Bump sbt-assembly to 0.14.7 for Scala 2.12

sbt-assembly 0.14.7 does not exist for Scala 2.12 and the build
would fail without a more recent version of the plugin.

Change-Id: I56f8714ee5c72632b8609b532ffb3b91d4466ad2

build.sbt

diff --git a/build.sbt b/build.sbt
index e59836d..e55957c 100644
--- a/build.sbt
+++ b/build.sbt
@@ -4,6 +4,8 @@
 
 version := GerritVersion + "-1"
 
+scalaVersion := "2.12.8"
+
 javacOptions ++= Seq("-source", "1.7", "-target", "1.7")
 
 libraryDependencies += ("com.google.gerrit" % "gerrit-plugin-api" % GerritVersion % "provided")

project/assembly.sbt

diff --git a/project/assembly.sbt b/project/assembly.sbt
index a815d58..d95475f 100644
--- a/project/assembly.sbt
+++ b/project/assembly.sbt
@@ -1 +1 @@
-addSbtPlugin("com.eed3si9n" % "sbt-assembly" % "0.14.0")
+addSbtPlugin("com.eed3si9n" % "sbt-assembly" % "0.14.7")