Explicit Node versions in Travis build matrix

This communicates more clearly for which versions of Node.js
the tests have run.

8 is not listed because tests are failing due to the nock HTTP mocking
library being incompatible with Node v8.

Author: Rob--W

.travis.yml

@@ -1,7 +1,9 @@
 language: node_js
 node_js:
-  - node
   - 0.10
+  - 4
+  - 6
+  - 7
 script:
   - npm run lint
   - npm run test