Make CI rebuild mocha.js

Make won't rebuild when just checked out by CI because timestamps on source and mocha.js are the same.

We could use a script to set the timestamps to the commit time, but it still wouldn't work if mocha.js were committed along with other changes that hadn't been built into it. Which shouldn't happen anyway -- but then again, the point of CI is to see what commits change, it's usually going to need to rebuild mocha.js if it's working right, so it's not going to hurt much to rebuild it every time even on the few times it doesn't need to.

.travis.yml

@@ -11,7 +11,7 @@ language: node_js
 matrix:
   include:
     - node_js: '6'
-      env: TARGET="lint test-node test-browser"
+      env: TARGET="clean lint test-node test-browser"
     - node_js: '5'
       env: TARGET=test-node
     - node_js: '4'