Reverted to sbt 1.2.8 because of travis *sigh*

bhermann · bhermann · commit 518f76a6644e · 2020-01-08T16:48:00.000+01:00
diff --git a/project/build.properties b/project/build.properties
@@ -1 +1 @@
-sbt.version = 1.3.6
+sbt.version = 1.2.8
