Fix `npm run-script coveralls`

The coveralls script was throwing an error, due to jscoverage being unable to process non-javascript files contained within the
nightwatch `lib` directory. The fix was to simply add the `--exclude *.ejs,*.json` flag to the invocation of jscoverage.
Merge pull request #163 from sethmcl/fix_jscoverage

Fix `npm run-script coveralls`