Add check for missing unidiff dependency

[thk123]

• Each commit message has a non-empty body, explaining why the change was made.
• [n/a] Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• [n.a] The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• [n/a] Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• [n/a] My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

This dependency isn't actually documented anywhere. Might as well document it at the source...

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 0e555e5).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/107982806