Allow to pass environment variables to the gem type

This is very useful when install gems like buildr that requires access to the JAVA_HOME environment variable:

```
rvm_gem {
  "${ruby193}@buildr/buildr":
    ensure => '1.4.7',
    environment => "JAVA_HOME=/usr/lib/jvm/java-6-openjdk/",
    require => [Rvm_gemset["${ruby193}@buildr"], Package["openjdk-6-jdk"]];
}
```