Update the minimum required version of gradle to 2.13

It came out with improvements around idea integration and language levels. This will make it possible to have the upcoming java client as a new project compiled against java 7 and have idea working on the right language level.

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")
             }