Bump sbt version to 0.13.9

There are several issues that report compilation errors with a message like:

    [error] impossible to get artifacts when data has not been loaded. IvyNode = com.fasterxml.jackson.core#jackson-databind;2.5.4
    java.lang.IllegalStateException: impossible to get artifacts when data has not been loaded. IvyNode = com.fasterxml.jackson.core#jackson-databind;2.5.4

Examples:

- linkedin#201
- linkedin#339
- linkedin#367
- linkedin#419
- linkedin#658

It looks like this message stems from a bug in Ivy which has since been fixed
(sbt/sbt#1598). I'm guessing the fix in Ivy is
included in sbt v0.13.9, because uprading sbt fixed the issue for me, and seems
to have helped others, too (e.g.,
linkedin#201 (comment)).

project/build.properties

@@ -14,4 +14,4 @@
 # the License.
 #
 
-sbt.version=0.13.5
+sbt.version=0.13.9