added support for release candidate versions of sbt

The PR enables support for Release Candidate versions of SBT. Such as this in the build.properties file:
```
sbt.version=0.13.5-RC1
```
This change has been tested with the following applications (as well as a few others):
https://github.com/naaman/spray-bootstrap
https://github.com/jkutner/sample-play-app

I was thinking we might want to restrict it to release candidates since, theoretically, they might be less likely to regress than a beta. but if we really don't care much about restricting the types of versions, then I see no problem loosening it.