Pin sbt version to prevent failures due to sbt upgrades

The build.properties specifying sbt version 0.13.x is needed to be sure
that the plugin is compiled via legacy  sbt assembly even after the
sbt 1.0 upgrade preventing sbt errors to appear.

Change-Id: I71a9d03dc0c8546dadfd1096f96db846cfc99f38
diff --git a/project/build.properties b/project/build.properties
new file mode 100644
index 0000000..64317fd
--- /dev/null
+++ b/project/build.properties
@@ -0,0 +1 @@
+sbt.version=0.13.15