Correct check mode for pip in virtualenv.

##### Issue Type:

Bugfix Pull Request for #412.

d3cb2d38b7d4b9585adfd4f1ceece1896c2cc952 added better support for check mode, but if the target is in a virtualenv, then it always returned `True`.  Since by that point in the code, we have already verified that we have a valid virtualenv, the pip freeze command will work correctly, so there should be no need to exit early.