Add activator.checkForUpdates property for disabling updates

Just don't go to the web for updates if this is set.
Make launcher generation available as a task

Then always write it to activator.properties so we no
longer have the default of 0 in code.

This will allow us to use the launcher generation when
generating the launcher config in Packaging.scala also.
Rename version.properties to include launcher generation

This means we ignore the previously-downloaded latest version
for a different generation.