Bump installed jenkins_master version

It looks like this version disappeared from the downloads page within
the past few weeks. I successfully ran this play less than a month ago,
but it started failing a week or two ago, due to the 404 upon download.

I'm admittedly a bit confused as to why there appears to be at least 3
different minor versions of jenkins in use [1][2][3] (plus a major bump
on the build server?! [4]), so I simply cribbed the version used by the
`tools_jenkins` role.

References:
- [1] jenkins_version: "1.638"
- [2] jenkins_tools_version: "1.651.3"
- [3] jenkins_admin_version: "1.658"
- [4] build_jenkins_version: jenkins_2.60.3

playbooks/roles/jenkins_master/defaults/main.yml

@@ -6,7 +6,7 @@ jenkins_port: 8080
 jenkins_nginx_port: 80
 jenkins_protocol_https: true
 
-jenkins_version: "1.638"
+jenkins_version: '1.651.3'
 jenkins_deb_url: "https://pkg.jenkins.io/debian-stable/binary/jenkins_{{ jenkins_version }}_all.deb"
 jenkins_deb: "jenkins_{{ jenkins_version }}_all.deb"
 # Jenkins jvm args are set when starting the Jenkins service, e.g., "-Xmx1024m"