Fixed garden install for newer virtualenvs

Switched from virtualenv-2.7 and pip-2.7 to virtualenv and pip. The -2.7 versions sometimes do not exist, but the plain versions always do. Plain virtualenv (system default virtualenv command) can be used because the version of Python is specified in the command, so it will always create a Python 2.7 venv even if it is created by virtualenv installed into Python2.6/3.3/etc.