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>

sponge-legacy/pom.xml

@@ -20,7 +20,7 @@
     <repositories>
         <repository>
             <id>sponge</id>
-            <url>http://repo.spongepowered.org/maven</url>
+            <url>https://repo.spongepowered.org/maven</url>
         </repository>
     </repositories>
 

sponge/pom.xml

@@ -19,7 +19,7 @@
     <repositories>
         <repository>
             <id>sponge</id>
-            <url>http://repo.spongepowered.org/maven</url>
+            <url>https://repo.spongepowered.org/maven</url>
         </repository>
     </repositories>