fix(config): webpack rc4 schema enforcment (fixes #193)

  - defaults webpackOptions.entry to a function

BREAKING CHANGE: Remove entry:{} from test configurations

When updating to `"webpack": "2.2.0-rc.4"` & `"karma-webpack": "1.8.2"` you have to pull the `entry` property if it's set to an empty object so it defaults to a function within karma-webpack

src/karma-webpack.js

@@ -30,7 +30,7 @@ function Plugin(
     // Webpack 2.1.0-beta.7+ will throw in error if both entry and plugins are not specified in options
     // https://github.com/webpack/webpack/commit/b3bc5427969e15fd3663d9a1c57dbd1eb2c94805
     if (!webpackOptions.entry) {
-      webpackOptions.entry = {}
+      webpackOptions.entry = function(){return {}}
     };
 
     if (!webpackOptions.output) {