Update badges
Test against RubyGems 2.2.2
Expand list of tested versions of Ruby
Remove obsolete gem-test functionality
Move gemspec out

Do this in order to prepare for simplified development using Bundler
approach with Gemfile
Use Bundler to install development dependencies
Update Travis to use Gemfile

Remove the individual before_script actions (only leave RubyGems
setup)
Move License out of README
Update minitest dependency

There are two warnings in RubyGems 1.8.x but we can live with that.
Loose rake dependency
Invert condition

Never liked unless .. else, really.
Stop using @installer

Local variables rules on readability, sorry.
Use RubyGems 2.2 specific to deal with extensions

Version 2.2.x puts compiled extensions *outside* Gem's directory,
resulting in the compiled artifacts not being available for bundling
the new gem.

Adjust those values to the specific temporary directory.

Patch by Eric Hodel.
rakefile.rb => Rakefile
Merge branch 'minor-cleanup'