Use env[:clearance] instead of env[:session]
Update documentation

Use more recent gem version that includes the integration tests
generator.
Speed up tests for apps using clearance with bcrypt

If clearance detects that it's being run in the test environment, it
will lower the bcyrpt cost factor (essentially, how much it will sleep)
to the minimum.
Update to the latest bundler version

There was an issue where Travis build was erring because the bundler
version was 1.2.3, and we explicitly upgrade rubygems via travis.yml.
Thus, the result was that rubygems 2.0 is no longer compatible with
older bundler versions. Thus let's always grab the latest version of
bundler for Travis builds.