Build: Upgrade min gradle to 3.3

CONTRIBUTING.md

@@ -88,8 +88,8 @@ Contributing to the Elasticsearch codebase
 **Repository:** [https://github.com/elastic/elasticsearch](https://github.com/elastic/elasticsearch)
 
 Make sure you have [Gradle](http://gradle.org) installed, as
-Elasticsearch uses it as its build system. Gradle must be version 2.13 _exactly_ in
-order to build successfully.
+Elasticsearch uses it as its build system. Gradle must be at least
+version 3.3 in order to build successfully.
 
 Eclipse users can automatically configure their IDE: `gradle eclipse`
 then `File: Import: Existing Projects into Workspace`. Select the

README.textile

@@ -200,7 +200,7 @@ We have just covered a very small portion of what Elasticsearch is all about. Fo
 
 h3. Building from Source
 
-Elasticsearch uses "Gradle":https://gradle.org for its build system. You'll need to have version 2.13 of Gradle installed.
+Elasticsearch uses "Gradle":https://gradle.org for its build system. You'll need to have at least version 3.3 of Gradle installed.
 
 In order to create a distribution, simply run the @gradle assemble@ command in the cloned directory.
 

buildSrc/build.gradle

@@ -23,8 +23,8 @@ apply plugin: 'groovy'
 
 group = 'org.elasticsearch.gradle'
 
-if (GradleVersion.current() < GradleVersion.version('2.13')) {
-  throw new GradleException('Gradle 2.13+ is required to build elasticsearch')
+if (GradleVersion.current() < GradleVersion.version('3.3')) {
+  throw new GradleException('Gradle 3.3+ is required to build elasticsearch')
 }
 
 if (JavaVersion.current() < JavaVersion.VERSION_1_8) {

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

@@ -122,7 +122,7 @@ class BuildPlugin implements Plugin<Project> {
             }
 
             // enforce gradle version
-            GradleVersion minGradle = GradleVersion.version('2.13')
+            GradleVersion minGradle = GradleVersion.version('3.3')
             if (GradleVersion.current() < minGradle) {
                 throw new GradleException("${minGradle} or above is required to build elasticsearch")
             }