Update karma-webpack depency to fix confusing error

Using karma-webpack-plugin 1.0.0 led to the following error from grunt
test immediately after generating.  I found the following fix in
karma-webpack that had not been included in karma-webpack-plugin because
it has since become karma-webpack.

error:
INFO [PhantomJS 1.9.7 (Mac OS X)]: Connected on socket 33J3dAUsvdMT6Io38nxm with id 51124214
PhantomJS 1.9.7 (Mac OS X) ERROR
  TEST RUN WAS CANCELLED because this file contains some errors:
      /.../eventlist/test/spec/components/EventlistApp.js

fix:
https://github.com/webpack/karma-webpack/commit/ad3be8452f4debd954256cffcb9ffd84f2b838d6
Merge pull request #28 from leafle/master

Update karma-webpack depency to fix confusing error