chore(build): record the version of node we expect.

The .nvmrc file is used by nvm if you don't specify a version. This lets us type Found '/Users/alexeagle/Projects/angular/.nvmrc' with version <0.12>
Now using node v0.12.2. It doesn't check that you are using the node version from the file, however, so this won't prevent version skew.

See https://github.com/creationix/nvm#usage

closes angular#1995

Author: alexeagle

.nvmrc

@@ -0,0 +1 @@
+0.12