Check if theres a BUILD file in gerrit-sbt-build-plugin.sh

Change-Id: I0f3a94883373bfc101b6e50d57d7941196a2bb82
diff --git a/jenkins/gerrit-sbt-build-plugin.sh b/jenkins/gerrit-sbt-build-plugin.sh
index a040278..d672add 100644
--- a/jenkins/gerrit-sbt-build-plugin.sh
+++ b/jenkins/gerrit-sbt-build-plugin.sh
@@ -1,6 +1,6 @@
 #!/bin/bash -e
 
-if [ "{branch}" == "master" ]
+if [ -f "gerrit/BUILD" ]
 then
   git read-tree -u --prefix=gerrit gerrit/{branch}
   . set-java.sh 8