Changed link on sign in button to just app.wercker.com

Everytime I clicked on the Sign in button, I'd reach a login form, irrespective of whether I was logged in or not.

By linking to just app.wercker.com, a logged in user will be taken to the app directly without login and a non-logged in user will be shown the login form.
Changed ruby to 1.9.3 in wercker.yml

Builds were failing because of mismatched versions.
Merge pull request #3 from rohitpaulk/master

Changed link on sign in button to just app.wercker.com