.travis.yml: Cache ~/.npm

This causes Travis to preserve the npm cache so that our testing is more resilient to npm registry hiccups.

Closes #53.

Author: jdalton

.travis.yml

@@ -1,5 +1,8 @@
 language: node_js
 node_js:
   - 6
+cache:
+  directories:
+    - $HOME/.npm
 after_script:
   - 'istanbul cover --report html node_modules/.bin/_mocha tests -- -u exports -R spec && codecov'