Refs #18. Use helper methods to make test code more readable and maintainable.
Refs #18. Lock virtualenv version when installing terrarium in test environment
Refs #18. Lock virtualenv version to supported versions, for now.
Refs #18. Test against each supported version of virtualenv
Refs #18. Use installed terrarium
Refs #18. Display command that is being executed
Refs #18. Added prefix to output
Refs #18. Call setup.py directly
Refs #18. Added TRAVIS_ prefix to env variables, to prevent conflict with virtualenvs option parsing. Note to future self: never do this again.