Update fom Scala 2.11.0-M3 to 2.11.0-M4 as a cross compiler.

Change-Id: If9b6dde6576c5b26d2e5a5044f4a28c6ee463af8
Switch from xsbt-gpg-plugin 0.6 to sbt-pgp 0.7.

https://groups.google.com/d/msg/simple-build-tool/Z54e0wM6SbU/9xrcC-YXyyMJ

Change-Id: Iffe259cfba9f8d3b4d84b75d1f2d77d0f783e17d
project/build.properties: new file to force the use of sbt 0.12.4.

If sbt is upgraded to 0.13.0 then the build fails, so force sbt to use
version 0.12.4.

Change-Id: Iae450a5fcdb81a5ead54d3dca8aeb6f8c2d9d26f