update fog gem to latest
removed old rake task until it's updated
enable thumbnailing on S3

- added url to optimized image model
- refactored s3_store & local_store
Update INSTALL-ubuntu.md

The removed paragraph was useful for a previous version of the "Updating Discourse" code above it. It seems useless now, as precise instructions are already in the code comments. In fact, it became incoherent, as we explicitily have "bluepill quit" in the code now.
Merge pull request #1276 from jbruni/patch-1

Update INSTALL-ubuntu.md
Merge pull request #1275 from ZogStriP/enable-thumbnailing-on-s3

Enable thumbnailing on s3