Replace string with existing variable in shell cmd

The default value of `composer_path` was hard coded into a shell
command.  This replaces the string with the respective variable.  Since
the command starts with a variable, the whole line is quoted as
[described](http://docs.ansible.com/playbooks_variables.html#hey-wait-a-yaml-gotcha)
in the Ansible docs.
Merge pull request #1 from tarleb/var-in-shell-cmd

Replace string with existing variable in shell cmd