Revert "Update the minimum required version of gradle to 2.13"

This reverts commit 8c6cbf0305156351a4d56950f96f7ff953d288eb. We need to update all our CI machines first, then get this change back in.

build.gradle

@@ -257,6 +257,7 @@ allprojects {
 
 idea {
   project {
+    languageLevel = org.elasticsearch.gradle.BuildPlugin.minimumJava.toString()
     vcs = 'Git'
   }
 }

buildSrc/src/main/groovy/org/elasticsearch/gradle/BuildPlugin.groovy

@@ -113,7 +113,7 @@ class BuildPlugin implements Plugin<Project> {
             }
 
             // enforce gradle version
-            GradleVersion minGradle = GradleVersion.version('2.13')
+            GradleVersion minGradle = GradleVersion.version('2.8')
             if (GradleVersion.current() < minGradle) {
                 throw new GradleException("${minGradle} or above is required to build elasticsearch")
             }