Fix crash in Mocha v6 due to undefined options

[almirfilho]

Passing undefined options to mocha v6 cause it to break.
This PR just prevent passing the options object like this:

{ 
  reporter: 'nyan',
  require: '@babel/register',
  grep: undefined,
  ui: undefined,
  timeout: undefined,
  invert: undefined,
  ignoreLeaks: undefined,
  growl: undefined,
  globals: undefined,
  colors: undefined,
  slow: undefined
}

And instead pass it like this:

{ 
  reporter: 'nyan',
  require: '@babel/register'
}

Tests passing. No side effects.
Thank you for this package!

[coveralls]

Coverage remained the same at 100.0% when pulling 8cdb5ae on almirfilho:master into 185d515 on pghalliday:master.

[almirfilho]

@pghalliday could you please merge this and bump a version? Thanks!

[bonesoul]

+1

[almirfilho]

This is no longer an issue since mocha version 6.1.0. Closing this PR.