upgrade to sbt 0.13.13

SethTisue · SethTisue · commit 2fd69b22f791 · 2016-11-29T15:02:17.000-08:00
this is a good thing in general, to be on a modern sbt version.

but also, in the Scala community build some weird dbuild issue
I don't understand is causing scala-xml to be built with 0.13.8
instead of being overridden by dbuild to 0.13.13 like all the
other projects. rather than mess with it, let's just upgrade.
diff --git a/project/build.properties b/project/build.properties
@@ -1 +1 @@
-sbt.version=0.13.8
+sbt.version=0.13.13

Original file line number	Diff line number	Diff line change
`@@ -1 +1 @@`
`1`		`-sbt.version=0.13.8`
	`1`	`+sbt.version=0.13.13`