Update development dependencies

Update RSpec to version 3. This deprecates the old, monkey-patching
`should` syntax in favor of the newer `expect` syntax, which all of
Crepe's specs have been updated to use.

Additionally, update Cane and Rake.

Signed-off-by: David Celis <me@davidcel.is>
Merge pull request #45 from davidcelis/update-dependencies

Update development dependencies