Fixed compatibility issue with grunt-mocha-istanbul

Version 1.1.1 added one simple bug

At travis build under node 0.8:

  npm ERR! peerinvalid The package mocha does not satisfy its siblings' peerDependencies requirements!
  npm ERR! peerinvalid Peer grunt-mocha-istanbul@1.2.0 wants mocha@*
  npm ERR! peerinvalid Peer mocha-sinon@1.1.1 wants mocha@^1.0.0

Reverting some version dep changes in package.json fixes issue