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.


This was a side of effective of diving into the sbt-launcher + taking with @harrah.   Not sure if it belongs against 1.0.x or somewhere else, but figured I'd send it off, since it's a 1-line patch and, for the most part, innocuous right now.

Review/decide by @havocp and @jamesward 

I tested locally and modifying `~/.activator/version.properties` does change the version of activator used.