Fixed issue with grunt build not completing correctly

Turns out that an issue in the older version of grunt-modernizr was causing grunt-build to not complete correctly. Upgrading from 0.3.1 to 0.4.1 seems to have fixed the issue.

Should fix #229 and #230.

For best results when testing:
```
rm -r node_modules
rm -r dist
npm install
grunt build
```