Fix bug when gem parameters are not a `Hash`

This fixes `TypeError` being raised whenever the parameters the gem has on a `Gemfile` are not a `Hash`.

**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` more evident by moving it to it's own private method.