Bump up scala to 2.11.12

Scala version 2.11.12 is the recommended version for compatibility to
java 11.

Bug: Issue 14376
Change-Id: I3733b4d874730954664f0508efa5e6da0dd40117
diff --git a/build.sbt b/build.sbt
index d681371..b6fb497 100644
--- a/build.sbt
+++ b/build.sbt
@@ -14,7 +14,7 @@
     resolvers += Resolver.mavenLocal,
     version := gerritApiVersion,
 
-    scalaVersion := "2.11.8",
+    scalaVersion := "2.11.12",
 
     libraryDependencies ++= Seq(
       "io.fabric8" % "gitective-core" % "0.9.54"