Create build.properties

Adding build.properties so SBT resolves the required version of SBT first and then it's able to assembly.
diff --git a/spot-ml/project/build.properties b/spot-ml/project/build.properties
new file mode 100644
index 0000000..b9b73f4
--- /dev/null
+++ b/spot-ml/project/build.properties
@@ -0,0 +1 @@
+sbt.version = 0.13.11