`ruby_bundle` should support Gemfiles with `gemspec` method

[kigster]

Currently bundle is created based on the Gemfile and Gemfile.lock only.

However, many custom gems use gemspec inside the Gemfile to delegate gem listing to the *.gemspec file.

bundle install works just the same, except it needs to have the gem's gemspec file present and at the same level as the Gemfile.

[kigster]

We still need this, but I am gonna ponder on this for now.