Upgrade mocha from 4.1.0 to 5.2.0

[cgewecke]

#132

• breaking changes in v5. (Looks fine)
• diff fix documented for 5.0.3 here.
• truffle recently upgraded to 5.2.0 here

v6.0.0 triggers an epilogue crash and needs investigation. Not obvious from the release notes why this might happen.

/home/travis/build/cgewecke/eth-gas-reporter/mock/node_modules/mocha/lib/reporters/base.js:319
  console.log(fmt, stats.passes || 0, milliseconds(stats.duration));
                         ^
TypeError: Cannot read property 'passes' of undefined

TBH mocha shouldn't be a dependency here but there have been strange issues with how mocha is scoped (locally vs. globally) relative to truffle, which ships with mocha as an un-webpacked dep. See #42.