[Fix #124] Move dependencies to gemspec and untrack Gemfile.lock

[paydaylight]

This is a duplicate of #247 because it will delete changes made in PR #246 and rebase and merge make no effect so I cherry-picked commits to this branch.

Before you submit a pull request, please make sure you have to follow:

• [x] read and know items from the Contributing Guide
• [x] add a description of the problem you're trying to solve (short summary from related issue)
• [ ] verified that cops are ordered by alphabet
• [ ] add a note to the style guide docs (if it needs)
• [x] add a note to the changelog file
• [x] the commit message contains the number of the related issue (if it presents) and word Fix if this PR closes related issue
• [x] squash all commits before submitting to review