1.1.3.1: Merge pull request #13 from msarahan/substitute_system_cmd

Fixes a bug where wrapped scripts would fail to run if env_setup_cmd was passed as None. This is true for Conda 3.18.3 calling Menuinst. Adds default "activate" string. This should be fixed in Conda, but we'll patch it here for now to not force a Conda release.