Ruby Version Fixes

[jeffreyc]

@christophermanning please review?

• Bumping .ruby_version to 2.3.0 for dev
• Setting up a matrix build of 2.0, 2.1, 2.2, and 2.3 in Travis
  • Travis's defaults for 2.1 and 2.2 are 2.1.5 and 2.2.0, thus the explicit bugfix versions
  • Travis doesn't recognize 2.3, thus the explicit bugfix version
  • The default bundler on Travis is 1.7.x, thus the explicit upgrade
• Setting the minimum Ruby version to 2.0 in the gemspec

[christophermanning]

LGTM