Update Maven references to Gradle

.github/CONTRIBUTING.md

@@ -6,9 +6,7 @@ forking the repository and sending a pull request.
 
 When submitting code, please make every effort to follow existing conventions
 and style in order to keep the code as readable as possible. Please also make
-sure your code compiles by running `mvn clean verify`. Checkstyle failures
-during compilation indicate errors in your style and can be viewed in the
-`checkstyle-result.xml` file.
+sure your code compiles by running `./gradlew build` (or `gradlew.bat build` on Windows).
 
 Before your code can be accepted into the project you must also sign the
 [Individual Contributor License Agreement (CLA)][1].

website/index.html

@@ -182,7 +182,7 @@ implementation 'com.squareup.retrofit2:retrofit:<span class="version pln"><em>(i
             <section id="contributing">
               <h3>Contributing</h3>
               <p>If you would like to contribute code you can do so through GitHub by forking the repository and sending a pull request.</p>
-              <p>When submitting code, please make every effort to follow existing conventions and style in order to keep the code as readable as possible. Please also make sure your code compiles by running <code>mvn clean verify</code>.</p>
+              <p>When submitting code, please make every effort to follow existing conventions and style in order to keep the code as readable as possible. Please also make sure your code compiles by running <code>./gradlew build</code> (or <code>gradlew.bat build</code> for Windows).</p>
               <p>Before your code can be accepted into the project you must also sign the <a href="https://squ.re/sign-the-cla">Individual Contributor License Agreement (CLA)</a>.</p>
             </section>