pry-rescue.gemspec: depend on interception v0.4 and higher

v0.4 adds Ruby 2.1.0 support.
bump version
Merge branch 'review.update-interception' of github.com:kyrylo/pry-rescue
bump version
Travis bundler caching
Merge pull request #61 from vincentwoo/travis-bundler-caching

Turn on travis bundler caching
.travis.yml: correct rbx

rbx-19mode is no more.
See: https://github.com/travis-ci/travis-ci/issues/1641#issuecomment-29009870