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

This reverts commit 0b965167fd.
All the CI machines have been updated to gradle 2.13, we should be ok upgrading the required minimum version.

build.gradle

@@ -257,7 +257,6 @@ 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('2.8')
+            GradleVersion minGradle = GradleVersion.version('2.13')
             if (GradleVersion.current() < minGradle) {
                 throw new GradleException("${minGradle} or above is required to build elasticsearch")
             }