Upgrade to the re-merged Node.js/io.js as the default.

Testing Done:
Locally `./pants clean-all test contrib/node::`

CI went green here:
  https://travis-ci.org/pantsbuild/pants/builds/79882306

Bugs closed: 2174

Reviewed at https://rbcommons.com/s/twitter/r/2800/

contrib/node/src/python/pants/contrib/node/subsystems/node_distribution.py

@@ -33,7 +33,7 @@ def register_options(cls, register):
       register('--supportdir', advanced=True, default='bin/node',
                help='Find the Node distributions under this dir.  Used as part of the path to '
                     'lookup the distribution with --binary-util-baseurls and --pants-bootstrapdir')
-      register('--version', advanced=True, default='0.12.7',
+      register('--version', advanced=True, default='4.0.0',
                help='Node distribution version.  Used as part of the path to lookup the '
                     'distribution with --binary-util-baseurls and --pants-bootstrapdir')