Pin mercurial plugin version

The newest version of the mercurial plugin was not compatible with
Jenkins 2.60.3.

Change-Id: I27961026a6bddbeac16137e1a8acf1316a5a720c
diff --git a/jenkins-docker/master/plugins.txt b/jenkins-docker/master/plugins.txt
index f9e2952..b334d6e 100644
--- a/jenkins-docker/master/plugins.txt
+++ b/jenkins-docker/master/plugins.txt
@@ -53,6 +53,7 @@
 mask-passwords:2.12.0
 matrix-auth:2.2
 matrix-project:1.13
+mercurial:2.6
 momentjs:1.1.1
 monitoring:1.73.0
 pipeline-build-step:2.7