Update the documentation to properly include jcenter() when disabling default repos

Without this, the user won't be able to resolve jruby-complete or jruby-mains

docs/news/2015-09-18-jruby-gradle-1-1.adoc

@@ -43,6 +43,8 @@ jruby {
 }
 
 repositories {
+    /* use jcenter() so we can get JRuby itself */
+    jcenter()
     /* use our new rubygems() function to add a rubygems repo */
     rubygems('https://rubygems.org')
 }

jruby-gradle-base-plugin/README.adoc

@@ -218,6 +218,7 @@ apply plugin: 'com.github.jruby-gradle.base'
 jruby.defaultRepositories false
 
 repositories {
+    jcenter()
     rubygems('https://rubygems.org')
 }