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>
diff --git a/docgen/pom.xml b/docgen/pom.xml
index b91563e..a03188f 100644
--- a/docgen/pom.xml
+++ b/docgen/pom.xml
@@ -35,7 +35,7 @@
       </snapshots>
       <id>jcenter-releases</id>
       <name>jcenter</name>
-      <url>http://jcenter.bintray.com</url>
+      <url>https://jcenter.bintray.com</url>
     </repository>
   </repositories>