Relax faraday gem dependency

* To avoid conflicts on faraday version bumps, allow faraday version to
  flex up to 0.8.x
* Remove the Gemfile.lock and ignore it, to let consumer apps figure out
  the dependency graph.
* Bump version from 3.5.2 to 3.5.3 to reflect these changes.
Merge pull request #37 from crashlytics/feature/relax-faraday-dependency

Relax faraday gem dependency