upgrade sbt to 0.13.15 (from 0.13.13)

SethTisue · SethTisue · commit 3a386ca3128a · 2017-04-10T17:11:42.000-05:00
diff --git a/project/build.properties b/project/build.properties
@@ -1 +1 @@
-sbt.version=0.13.13
+sbt.version=0.13.15
