Revert "Travis CI: Specify node.js 5.6 since 5.7.0 has regressions."

This reverts commit d0059ac2ed.

Node.js 5.7.1 is out with the issue fixed.

[skip sauce]

.travis.yml

@@ -5,7 +5,7 @@ git:
   depth: 10
 node_js:
   - "4"
-  - "5.6"
+  - "5"
 before_install:
   # Remove ./node_modules/.bin from PATH so node-which doesn't replace Unix which and cause RVM to barf. See https://github.com/travis-ci/travis-ci/issues/5092
   - export PATH=$(python -c 'from sys import argv;from collections import OrderedDict as od;print(":".join(od((p,None) for p in argv[1].split(":") if p.startswith("/")).keys()))' "$PATH")