Specify branch for pulling ruby-build

New jenkins nodes comes with newer git with strict rules
for branch operations (or it just comes without set-upstream-to)

PROD-35466

Change-Id: I6be44c85e826d4da23a02353f94c17f109cec622
diff --git a/src/com/mirantis/mk/Ruby.groovy b/src/com/mirantis/mk/Ruby.groovy
index 8fac324..4681d07 100644
--- a/src/com/mirantis/mk/Ruby.groovy
+++ b/src/com/mirantis/mk/Ruby.groovy
@@ -11,7 +11,7 @@
 def ensureRubyEnv(rubyVersion="2.4.1"){
     if (!fileExists("/var/lib/jenkins/.rbenv/versions/${rubyVersion}/bin/ruby")){
         //XXX: patch ruby-build because debian package is quite old
-        sh "git clone https://github.com/rbenv/ruby-build.git ~/.rbenv/plugins/ruby-build || git -C ~/.rbenv/plugins/ruby-build pull"
+        sh "git clone https://github.com/rbenv/ruby-build.git ~/.rbenv/plugins/ruby-build || git -C ~/.rbenv/plugins/ruby-build pull origin master"
         sh "rbenv install ${rubyVersion} -sv";
     }
     sh "rbenv local ${rubyVersion};rbenv exec gem update --system"