Depend on rspec 3
Migrate .rspec options into ruby config
Enable both should and expect syntax for now
Remove pending sort spec as it got implemented recently
Fix usage of `be_true` and `be_false` for rspec 3

They now become `be true` and `be false` respectively, otherwise rspec expects the object to respond to `#true?` or `#false?`.
Merge pull request #112 from guilleiguaran/rspec-3

RSpec 3 support (& fix tests under rbx-2)

Fixes #108 and #111