update out-of-date dev-dependency according to npm audit security report

[nishch]

Update outdated mocha dev-dependency to fix npm security audit warnings

This PR fixes the open issue #240

Checklist

• I have read the CONTRIBUTING guidelines.
• I have added test cases which verify the correct operation of this feature or patch.
• I have added documentation pertaining to this feature or patch.
• The automated test suite ($ make test) executes successfully.
• The automated code linting ($ make lint) executes successfully.

[coveralls]

Coverage remained the same at 99.79% when pulling 747cf81 on nishch:master into 9de7d64 on jaredhanson:master.