Use HTTPS instead of HTTP to resolve dependencies

This fixes a security vulnerability in this project where the `pom.xml`
files were configuring Maven to resolve dependencies over HTTP instead of
HTTPS.

Signed-off-by: Jonathan Leitschuh <Jonathan.Leitschuh@gmail.com>

examples/gradle/pom.xml

@@ -18,7 +18,7 @@
     <pluginRepositories>
         <pluginRepository>
             <id>jcenter</id>
-            <url>http://jcenter.bintray.com/</url>
+            <url>https://jcenter.bintray.com/</url>
         </pluginRepository>
     </pluginRepositories>