Enable karma-sourcemap-loader

This turns (some) stacktraces back into something useful, so that unpicking
test failures is vaguely tractable

karma.conf.js

@@ -84,7 +84,7 @@ module.exports = function (config) {
         // available preprocessors:
         // https://npmjs.org/browse/keyword/karma-preprocessor
         preprocessors: {
-            '{src,test}/**/*.js': ['webpack'],
+            '{src,test}/**/*.js': ['webpack', 'sourcemap'],
         },
 
         // test results reporter to use

package.json

@@ -119,6 +119,7 @@
     "karma-cli": "^0.1.2",
     "karma-junit-reporter": "^0.4.1",
     "karma-mocha": "^0.2.2",
+    "karma-sourcemap-loader": "^0.3.7",
     "karma-webpack": "^1.7.0",
     "matrix-mock-request": "^1.2.0",
     "matrix-react-test-utils": "^0.2.0",