Modified peer dependency to mocha to allow all future versions.

  The version for the mocha peer dependency was specified as "~1.20.1"
  which does not allow any versions of mocha from 1.21.0 and up to be
  installed alongside mocha-given. This update allows all future
  versions of mocha to be installed together with mocha-given.

  Defining peer dependencies in this way seems to be the de facto
  standard.