Use HTTPS instead of HTTP to resolve dependencies (#183)

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>

sponge/pom.xml

@@ -54,7 +54,7 @@
         <repository>
             <id>sponge-maven-repo</id>
             <name>Sponge maven repo</name>
-            <url>http://repo.spongepowered.org/maven</url>
+            <url>https://repo.spongepowered.org/maven</url>
             <releases>
                 <enabled>true</enabled>
             </releases>