Fix browser test suite by using Node.js 6 (#667)

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.

For a long time, we've been installing npm v2.x before running tests
on Travis CI across all Node.js versions.

While doing some testing today, I observed that we were able to install
dependencies and run all tests just fine on all Node.js versions, except
for v0.8. Therefore ensure we only force an install of npm v2.x when
running tests on Node.js 0.8, and keep the installed npm version as is
for all other Node.js versions.

.travis.yml

@@ -4,11 +4,12 @@ node_js:
   - '0.10'
   - 0.12
   - 4
+  - 6
 before_install:
-  - npm install -g npm@2.x
+  - "test $TRAVIS_NODE_VERSION != '0.8' || 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=