Add model helpers to fetch reverse dependency gems

These helpers provide a way to collect versions or rubygems which depend
on a specific rubygem. They can be used to query the database for all the
gems which contain a given dependency.
Reuse a helper method to dry the code
Add 'reverse_dependencies' controller action
Provide tests for reverse_dependencies model helpers
Add full_name field to the reverse_dependencies payload
Add reverse_dependencies action for rubygems, shorter output version
Update ffi dependency to version 1.5.0
Update newrelic_rpm dependency to version 3.6.0.78
Update nokogiri dependency to version 1.5.9
Update shoulda-matchers dependency to version 1.5.4
Update multi_json dependency to version 1.7.2
Update faraday dependency to version 0.8.7
Merge pull request #540 from alup/master

[feature] List reverse dependencies for a specific gem
Add CONTRIBUTING.md file

Consolidated two wiki pages into this contributing guidelines
page:
    - Contributing Guidelines
    - Development Setup
Merge pull request #549 from parkr/patch-1

Add CONTRIBUTING.md file
Emdash trace is totally not a thing.
Merge pull request #550 from parkr/TRACE-OH-NOES

Emdash trace is totally not a thing.