Specify redis gem version for Ruby 2.2

[tobyhs]

Goal

Fix the Travis build for Ruby 2.2 and Sidekiq. I also started using Gem::Version as that seems to be more accurate, especially when version components are more than one digit.

Review

For the submitter, initial self-review:

• Commented on code changes inline explain the reasoning behind the approach
• Reviewed the test cases added for completeness and possible points for discussion
• A changelog entry was added for the goal of this pull request
• Check the scope of the changeset - is everything in the diff required for the pull request?
• This pull request is ready for:
  • Initial review of the intended approach, not yet feature complete
  • Structural review of the classes, functions, and properties modified
  • Final review

For the pull request reviewer(s), this changeset has been reviewed for:

• Consistency across platforms for structures or concepts added or modified
• Consistency between the changeset and the goal stated above
• Internal consistency with the rest of the library - is there any overlap between existing interfaces and any which have been added?
• Usage friction - is the proposed change in usage cumbersome or complicated?
• Performance and complexity - are there any cases of unexpected O(n^3) when iterating, recursing, flat mapping, etc?
• Concurrency concerns - if components are accessed asynchronously, what issues will arise
• Thoroughness of added tests and any missing edge cases
• Idiomatic use of the language

[tobyhs]

It looks like the branch build for this passed (after I retried many times). I'm now encountering the same rvm issues with the pull request build on the jruby-19mode job (where running rvm get stable tries to download https://github.com/rvm/rvm/archive/.tar.gz ; the branch/tag/version is missing) that doesn't appear to be related to this change (as the failure also comes up many times on a whitespace change).