Add .ruby-version file to .gitignore
Use https://rubygems.org as the gem source.
Don't require Psych to be present.
Drop Ruby 1.9.2 support.
Remove Travis CI images.

Travis CI is too unreliable -- builds die all the time for
reasons that are outside my control.
Add back build status icons.
Don't ignore Gemfile lock files.
Use more precise gem dependencies.

And add Gemfile.lock file.
Update supported Rubies list.