mavengem-wagon needs to be in .mvn/extensions.xml for some stuff

Seems like this is needed for it to be picked up by some of the
integration tests; specifically, the bundler-maven-plugin ITs fail
if this is not present. Perhaps missed during the update to
mavengem originally.

Author: headius

.mvn/extensions.xml

@@ -0,0 +1,8 @@
+<extensions xmlns="http://maven.apache.org/EXTENSIONS/1.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://maven.apache.org/EXTENSIONS/1.0.0 http://maven.apache.org/xsd/core-extensions-1.0.0.xsd">
+  <extension>
+    <groupId>org.jruby.maven</groupId>
+    <artifactId>mavengem-wagon</artifactId>
+    <version>2.0.2</version>
+  </extension>
+</extensions>