try mirroring rubygems repo

jruby/jruby-maven-plugins#100 (comment)

Author: maybeec

.m2/settings.xml

@@ -0,0 +1,13 @@
+<settings xmlns="http://maven.apache.org/SETTINGS/1.0.0"
+  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://maven.apache.org/SETTINGS/1.0.0
+                      https://maven.apache.org/xsd/settings-1.0.0.xsd">
+  <mirrors>
+    <mirror>
+      <id>rubygems-releases-mirror</id>
+      <name>RubyGems Releases mirror</name>
+      <url>https://repo.maven.apache.org/maven2</url>
+      <mirrorOf>rubygems-release</mirrorOf>
+    </mirror>
+  </mirrors>
+</settings>