Update install instruction to install grunt-mocha globally

Otherwise goes away when you blast webapp dir (like in the case
of testing the generator...)
Revert "Merge pull request #59 from fayimora/improve-testing"

On `grunt test` getting:

Running "mocha:all" (mocha) task
Testing: http://localhost:9000/index.html
...ERROR
Warning: PhantomJS unable to load "http://localhost:9000/index.html"
URI. Use --force to continue.

This reverts commit 4f241d7ecf26cb7d88c5823778333ad3b935558e, reversing
changes made to 5ac17455d00c268b444f2a590608a3c786acdc3b.
0.4.1