Make run  command working

The `install_executable` task installs composer inside the shared folder within the name of `composer.phar` which is a php packages without execute rights. So when `run` command runs it ends up with an error like `composer: command not found`. The problems are:

- composer.phar is the file name not composer. 
- since the file is not executable you can't execute it

This PR solves the file name and makes composer executed through php.