Add mocha to eslintrc environment to support describe

[pmcilwaine]

This ensures mocha global variables such as describe do not fail eslint

[coveralls]

Coverage remained the same at 96.429% when pulling 26ed911525d1fac513e33559b4872c3da4e7fc11 on pmcilwaine:feature/update-eslintrc into edb30419d583c5c2d1f36686b33011c038a80af2 on rundef:master.