Minor refactor
* Renamed methods to better represent the logic.
* Removed `name` param from `Railblazer::Gem#new` since it was not needed.
* Made `Railblazer::MinimalGemfile` dependency on `Railblazer::Gem` by moving to it's own private method.
Fix bug when gem parameters are not a `Hash`
This fixes `TypeError` being raised whenever the parameters the a gem has on a `Gemfile` are not a `Hash`.

This changes allow fixes this type of lines from raising errors:
`gem 'shoulda-matchers', '~> 1.3.0'`
Merge pull request #10 from sebasoga/master

Fix bug when gem parameters are not a `Hash`