Replace `be_true`/`be_false` with `be_truthy`/`be_falsey`

Because `be_true` and `be_false` were deprecated in rspec v2.99, see
https://github.com/rspec/rspec-expectations/blob/master/Changelog.md#2990beta1--2013-11-07
Merge pull request #38 from mocoso/be_truthy

Replace `be_true`/`be_false` with `be_truthy`/`be_falsey`