Don't delete system-installed base jar on build

To save space in toplogy jars, the CLI deletes the unnecessary base jar
from the virtualenv before packing it into the topology jar.

This didn't account for users using --system-site-packages, however, and
it would still attempt to delete the base jar from its system-installed
location.

Closes #74
Merge pull request #78 from patricklucas/dont_delete_system_base_jar

Don't delete system-installed base jar on build