Update rbenv_gem to allow for local .gem installs

I believe this is the change your requested in #47.

A new code path is introduced where we detect a :source option that ends
with ".gem" and don't include any "--source" arguments in our call to
the gem command. This has the intended behavior of installing the .gem
file passed as source.

For example:
```
rbenv_gem 'god' do
  ruby_version '2.0.0-p353'
  source '/tmp/god-0.13.4.gem'
end
```

Also took the opportunity to update the --source line from http:// to https://