runTask: Escape backslashes

vars/runTask.groovy

@@ -50,7 +50,7 @@ def call(String task, String branch, String repo, String phpversion, String data
 
             stage("Detect Moodle Version") {
                 steps {
-                    moodleversion = sh grep '$branch' moodle/version.php | sed "s/^[^']*'\([^']*\).*$/\1/"
+                    moodleversion = sh grep '$branch' moodle/version.php | sed "s/^[^']*'\\([^']*\\).*$/\\1/"
                     runner = "${env.PATHTORUNNER}/${moodleversion}"
                     if (!fileExists(runner)) {
                         runner  "${env.PATHTORUNNER}/master"