travis: use npm test only. npm install attempts to download prebuilt node-pre-gyp binary (#450)

Author: ksmyth

.travis.yml

@@ -22,3 +22,7 @@ sudo: false
 before_install:
   - $CXX --version
   - if [ "$TRAVIS_NODE_VERSION" = "0.8" ]; then npm install -g [email protected]; fi;
+
+install: true
+
+script: npm test