Update sbt-extras to support Scala 2.10.0 by default

Scala 2.10.0 has been officially released :cake: and latest version of sbt-extras now refer to it with option `-210` (see paulp/sbt-extras@d074c7bd4520d33494609f73fc132612bf0d5c17)

### Additional Notes:

 * This pull request is an update to #107
 * I noticed that `sbt-extras` is still not (properly) installed on the JVM worker boxes  (see details history in #107). It will be a great improvement if Travis team could make it live as soon as possible :dancer:! 
 * I'll try my best to help, if needed... feel free to ask.

(Sorry for abusing of icons, I just discovered them right now, and I find it so cuuuute :baby: . But let's keep some ASCII in GitHub wonderworld: `<3 <3 <3` to you travis guys!)