Only install npm v2 on Travis CI when testing Node.js 0.8

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 versio nas is
for all other Node.js versions.

.travis.yml

@@ -6,7 +6,7 @@ node_js:
   - 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 != '6' || npm run test-browser"