Allow activator version to be pulled from ~/.activator/verison.properties.

This enables auto-upgrading if we want to finish wiring things through.
The basic gist is:

1. Detect if we're on the latest version of activator
2. If not, bump the version in version.properties
3. "Reboot" the application into the version found in version.properties.

This at least gets the initial startup correct, but not the live
rebooting.
Merge pull request #117 from typesafehub/wip-auto-upgrade

Allow activator version to be pulled from ~/.activator/verison.properties