upgrade sbt from 0.13.7 to 0.13.9

SethTisue · SethTisue · commit cc05d611ec09 · 2015-09-14T15:08:35.000-04:00
because why not, the sbt build is experimental anyway
diff --git a/project/build.properties b/project/build.properties
@@ -1 +1 @@
-sbt.version=0.13.7
+sbt.version=0.13.9
