Fix bug with vendored assets

Add #filter_gemspecs to `GemRelease::Helpers` to remove gemspecs listed
that exist in Bundler's vendor folder.

Add specs for following cases:
- No vendor folder gemspecs
- Mix of vendor folder gemspecs and other gemspecs
- Gemspecs existing inside a folder such as `src/vendor` which should be
accessible to `gem-release`.

Add vendor folder to gitignore.
Remove duplication with setup block in helpers_test

Extract `ReleaseCommand.new` into a `setup` block to remove duplication.