Remove CasperJS from the GeoExt repository only installing it via NPM if needed.

These changes aimed to remove unrelated files from the GeoExt repository by removing the duplicated CasperJS repository. Readme.md, .travis.yml and .gitignore have been updated accordingly to make sure everything still works as expected.

I hope I'm not intruding on other build mechanics that are not apparent via the repo. If these changes are a no-go, I'm ok with it.