Fix tests on Ruby 1.8

See #713.
Test against JRuby in Ruby 2.0-mode and Ruby 2.1-mode
Add support for Rubinius 2

See #713.
Test against Ruby 2.1.0

Closes #713.
Remove circular requires
Fail slow
Add RuboCop
Fix RuboCop offenses
Enforce code coverage minimums
Use gender-neutral example name
Use RSpec expectation syntax
Bump version to 1.2.0
Specify rack ~> 1.0 dependency
Include all files committed to the git repo
Bump version to 1.2.1
Fix RuboCop violations in gemspec
Fix RuboCop offenses
Update Travis settings for Rubinius

/cc @brixen
Use resolution-independent SVG badges [ci skip]
Stop signing gem [ci skip]