Use ssh for scm.developerConnection

Plugin releases are pushed by `mvn release:perform` using the
scm.developerConnection.  I don't want to switch my push authentication
method from ssh to https for plugins that I release from the command line.
Switch back to ssh authentication until this plugin uses automated release.

pom.xml

@@ -55,7 +55,7 @@
 
   <scm>
     <connection>scm:git:https://github.com/${gitHubRepo}</connection>
-    <developerConnection>scm:git:https://github.com/${gitHubRepo}</developerConnection>
+    <developerConnection>scm:git:git@github.com:${gitHubRepo}.git</developerConnection>
     <tag>${scmTag}</tag>
     <url>https://github.com/${gitHubRepo}</url>
   </scm>