Convert DEPS to only reference Git repos

Also replace all doublequoted Python strings
with single-quoted ones.

BUG=chromium:412012
R=phoglund@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/30289004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7793 4adac7df-926f-26a2-2b94-8c16560cd09d