Build: Require exactly gradle 2.13

see #18935

buildSrc/build.gradle

@@ -23,6 +23,16 @@ apply plugin: 'groovy'
 
 group = 'org.elasticsearch.gradle'
 
+// TODO: remove this when upgrading to a version that supports ProgressLogger
+// gradle 2.14 made internal apis unavailable to plugins, and gradle considered
+// ProgressLogger to be an internal api. Until this is made available again,
+// we can't upgrade without losing our nice progress logging
+// NOTE that this check duplicates that in BuildPlugin, but we need to check
+// early here before trying to compile the broken classes in buildSrc
+if (GradleVersion.current() != GradleVersion.version('2.13')) {
+  throw new GradleException('Gradle 2.13 is required to build elasticsearch')
+}
+
 if (project == rootProject) {
   // change the build dir used during build init, so that doing a clean
   // won't wipe out the buildscript jar