Merge pull request #653 from exercism/rename-repo

Remove leading 'x' from repository name

config.json

 {
   "slug": "java",
   "language": "Java",
-  "repository": "https://github.com/exercism/xjava",
+  "repository": "https://github.com/exercism/java",
   "active": true,
   "exercises": [
     {