Updated documentation to include precise version of gradle currently required for building (#20776)

CONTRIBUTING.md

@@ -88,7 +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.
+Elasticsearch uses it as its build system. Gradle must be version 2.13 _exactly_ 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 a modern version of Gradle installed - 2.13 should do.
+Elasticsearch uses "Gradle":https://gradle.org for its build system. You'll need to have version 2.13 of Gradle installed.
 
 In order to create a distribution, simply run the @gradle assemble@ command in the cloned directory.