Why display "Your requirements could not be resolved to an installable set of packages."

I was trying to install "shift31/laravel-elasticsearch":"~1.0" in laravel 5.1 but it give me a following error

Below is the result I get after running composer update:

Problem 1
- shift31/laravel-elasticsearch 1.3 requires elasticsearch/elasticsearch ~1.3 -> satisfiable by elasticsearch/elasticsearch[v1.3.0, v1.3.1, v1.3.2, v1.3.3, v1.3.4, v1.4.0, v1.4.1].
- shift31/laravel-elasticsearch 1.3.1 requires elasticsearch/elasticsearch ~1.3 -> satisfiable by elasticsearch/elasticsearch[v1.3.0, v1.3.1, v1.3.2, v1.3.3, v1.3.4, v1.4.0, v1.4.1].
- shift31/laravel-elasticsearch 1.3.2 requires elasticsearch/elasticsearch ~1.3 -> satisfiable by elasticsearch/elasticsearch[v1.3.0, v1.3.1, v1.3.2, v1.3.3, v1.3.4, v1.4.0, v1.4.1].
- shift31/laravel-elasticsearch 1.3.3 requires elasticsearch/elasticsearch ~1.3 -> satisfiable by elasticsearch/elasticsearch[v1.3.0, v1.3.1, v1.3.2, v1.3.3, v1.3.4, v1.4.0, v1.4.1].
- shift31/laravel-elasticsearch 1.3.4 requires elasticsearch/elasticsearch ~1.3 -> satisfiable by elasticsearch/elasticsearch[v1.3.0, v1.3.1, v1.3.2, v1.3.3, v1.3.4, v1.4.0, v1.4.1].
- shift31/laravel-elasticsearch 1.2 requires elasticsearch/elasticsearch ~1.2 -> satisfiable by elasticsearch/elasticsearch[v1.2.0, v1.2.1, v1.2.2, v1.3.0, v1.3.1, v1.3.2, v1.3.3, v1.3.4, v1.4.0, v1.4.1].
- shift31/laravel-elasticsearch 1.1.1 requires elasticsearch/elasticsearch ~1.1 -> satisfiable by elasticsearch/elasticsearch[v1.1.0, v1.2.0, v1.2.1, v1.2.2, v1.3.0, v1.3.1, v1.3.2, v1.3.3, v1.3.4, v1.4.0, v1.4.1].
- shift31/laravel-elasticsearch v1.1 requires elasticsearch/elasticsearch ~1.1 -> satisfiable by elasticsearch/elasticsearch[v1.1.0, v1.2.0, v1.2.1, v1.2.2, v1.3.0, v1.3.1, v1.3.2, v1.3.3, v1.3.4, v1.4.0, v1.4.1].
- elasticsearch/elasticsearch v1.4.1 requires ext-curl * -> the requested PHP extension curl is missing from your system.
- elasticsearch/elasticsearch v1.4.0 requires ext-curl * -> the requested PHP extension curl is missing from your system.
- elasticsearch/elasticsearch v1.3.4 requires ext-curl * -> the requested PHP extension curl is missing from your system.
- elasticsearch/elasticsearch v1.3.3 requires ext-curl * -> the requested PHP extension curl is missing from your system.
- elasticsearch/elasticsearch v1.3.2 requires ext-curl * -> the requested PHP extension curl is missing from your system.
- elasticsearch/elasticsearch v1.3.1 requires ext-curl * -> the requested PHP extension curl is missing from your system.
- elasticsearch/elasticsearch v1.3.0 requires ext-curl * -> the requested PHP extension curl is missing from your system.
- elasticsearch/elasticsearch v1.2.2 requires ext-curl * -> the requested PHP extension curl is missing from your system.
- elasticsearch/elasticsearch v1.2.1 requires ext-curl * -> the requested PHP extension curl is missing from your system.
- elasticsearch/elasticsearch v1.2.0 requires ext-curl * -> the requested PHP extension curl is missing from your system.
- elasticsearch/elasticsearch v1.1.0 requires ext-curl * -> the requested PHP extension curl is missing from your system.
- Installation request for shift31/laravel-elasticsearch ~1.0 -> satisfiable by shift31/laravel-elasticsearch[1.1.1, 1.2, 1.3, 1.3.1, 1.3.2, 1.3.3, 1.3.4, v1.1].

To enable extensions, verify that they are enabled in those .ini files: <br />
- /etc/php5/cli/php.ini <br />
- /etc/php5/cli/conf.d/05-opcache.ini <br />
- /etc/php5/cli/conf.d/10-pdo.ini <br />
- /etc/php5/cli/conf.d/20-json.ini <br />
- /etc/php5/cli/conf.d/20-mcrypt.ini <br />
- /etc/php5/cli/conf.d/20-mysql.ini <br />
- /etc/php5/cli/conf.d/20-mysqli.ini <br />
- /etc/php5/cli/conf.d/20-pdo_mysql.ini <br />
- /etc/php5/cli/conf.d/20-readline.ini <br />

You can also run php --ini inside terminal to see which files are used by PHP in CLI mode.

via Chebli Mohamed