Don’t serve dependencies that are already bundled

Because the test modules are passed through Browserify, all their dependencies
are already part of the resulting `tests.js` bundle file, just like the tested
application code.

testem.json

@@ -1,8 +1,6 @@
 {
   "framework": "mocha",
   "src_files": [
-    "node_modules/chai/chai.js",
-    "node_modules/sinon/pkg/sinon.js",
     "build/tests.js"
   ],
   "launchers": {