Migrate to RSpec's new expect syntax using Transpec.

We set `config.infer_spec_type_from_file_location!`, so the command I
ran is `transpec --no-explicit-spec-type`.
Upgrade RSpec to 3.0.0.
Follow the deprecation warnings after upgrading RSpec to 3.
Merge pull request #560 from knu/rspec3

Migrate to Rspec3.