Fix #80 - Uploader generates wrong key when override #default_url
Fix glitch: starts-with policy is not strict enough

A user can upload and overwrite another's avatar by changing guid part of key input in the form.
ignore .ruby-version
Merge branch 'master' of github.com:dwilkie/carrierwave_direct
Merge branch 'master' of git://github.com/dunghuynh/carrierwave_direct into dunghuynh-master