Add integration tests

Adds an initial battery of integration tests. 

They basically run `set-env.bat` and then check:
* if the environment is properly configured
* if the expected commands are available in the expected version
* if the aliases are defined
* if the gems / knife-plugins / vagrant plugins are installed in the expected ruby installations

These are the first 50% of #4, acceptance-level tests are still to come.

Also removed the travis-ci build since there are currently no useful tests we could run on travis (we need a windows environment :-/)