Fix type of launcher generation check

This worked because for now we don't have a launcher
generation on typesafe.com so we would fall back to 0
which was an integer. But if we'd added a launcher generation,
we would have checked int-vs-string equality which would
have always failed.

Score one point in the case for a typesafe == I guess.
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.
Fix test for whether to check updates

Parentheses fail.
Don't use read() to define boot properties file

Since read gets stuff from the boot properties
Update README to suggest activator.checkForUpdates=false
Merge pull request #320 from typesafehub/wip/havocp-ignore-other-gen-version-props

Ignore version.properties from other generations