Fix bash error when free command does not exist, #1367

[sdondley]

This PR fixes #1376 by fixing logic for handling a non-existent "free" command on the system.

Checklist

• [X] based on top of latest source code
• [ ] changelog entry included
• [X] tests pass on Travis CI
• [X] git history is clean
• [X] git commit messages are well-written