Adds support for Scala 2.12

The first commit is straight forward, it just sets new version.

The second one is a bit more problematic. The change is to depend on the 'new' scala-xml bundle, instead of only using the package dependency. This is required to be able to load scalariform and multiple versions of Scala in the same OSGi container, like we want to do in Scala IDE. 
If an OSGi bundle requires Scala 2.11 and the package scala.xml, and Scala 2.10 is also available, the OSGi container will decide that the bundle depends on both Scala 2.11 and 2.10, which is is impossible.
The practical problem is that this change is not compatible with building scalariform on Scala 2.10.

In Scala IDE, we solve the problem by adding a [pre-step](https://github.com/scala-ide/scala-ide/blob/809cc76494603192e8452d13d5fc5b48ce1ae977/build-all.sh#L26) to copy the [right file for the version of Scala](https://github.com/scala-ide/scala-ide/tree/809cc76494603192e8452d13d5fc5b48ce1ae977/org.scala-ide.sdt.core/resources/META-INF). Unfortunately, we haven't found a better solution, mostly due to an abuse of the Maven lifecycle by Tycho (the Eclipse build maven plugin).