Use RSpec expect syntax where Rubinius demands it

Rubinius does not allow binding Kernel methods to BasicObject instances (which
seems correct), so RSpec's `should` syntax breaks for collection objects, which
descend from BasicObject. The newer RSpec expect() syntax works fine though. In
general prefer expect() henceforth but it's not worth going through every test
to change it right now.
Testing powerups

Use the Travis configuration for local Rubies as well. Test against MRI 2.1, 2.0,
1.9; Rubinius 2.2
Merge pull request #67 from outoftime/expect

Ruby 2.1 and Rubinius 2.2