Configurable environment variables for composer:install

(replaces #5)

I am working on rewriting [Capifony to v3](https://github.com/everzet/capifony/pull/437), and rather than rewriting composer logic there, it is better to use this gem.

We need to be able to set environment variables during the composer run therefore I have added that capability

Therefore a user can do something like 

```ruby
set :composer_env, { symfony_env: 'prod' } 
```

Which would result in `SYMFONY_ENV=prod /usr/bin/env composer install` being run