Fix browser tests by running them on Node.js 6

For a long time we've struggled getting the browser test suite to run
on sauce labs. Those tests were running on Node.js 4 which is getting
quite old.

In a trial-n-error effort, I tried adding Node.js 6 as one of the
versions to run on Travis CI, and at the same time ensure the browsers
tests were run on that version instead of Node.js 4 -- voila the suite
passed right away.

It therefore seemed like the issues we were seeing had to do with us
running those tests via Node.js 4.

.travis.yml

@@ -4,11 +4,12 @@ node_js:
   - '0.10'
   - 0.12
   - 4
+  - 6
 before_install:
   - npm install -g npm@2.x
 script:
   - npm test
-  - "test $TRAVIS_PULL_REQUEST != 'false' || test $TRAVIS_NODE_VERSION != '4' || npm run test-browser"
+  - "test $TRAVIS_PULL_REQUEST != 'false' || test $TRAVIS_NODE_VERSION != '6' || npm run test-browser"
 env:
   global:
   - secure: L0dg0jr2fwkc2tPwP5svybILPBn2qdLzMrWc5tEXg3MPcy8D59Gvf+ri7INqo+ETPM20o5CsaDCH+LHUNS/V0G4VG1ajvsy7d8uh3hnb/K6VfVui/CjsHIqOcOZrbxVxgyX+iMXEXAj0+Syow9uDQHVhrz1qqad1n79likNCXa4=