Remove use of django 1.4 timezone helpers
Remove use of django 1.4 timezone helpers

Closes #400

evernote/graphite-web,Krylon360/evernote-graphite-web,Krylon360/evernote-graphite-web,Krylon360/evernote-graphite-web,Krylon360/evernote-graphite-web,evernote/graphite-web,Krylon360/evernote-graphite-web,evernote/graphite-web,Krylon360/evernote-graphite-web,evernote/graphite-web,evernote/graphite-web