Update default Rakudo-Star version to 2018.06

NOTE: Since there was no 2018.07 release of Rakudo, 2018.06 was used as the
basis for this Star release instead.

See https://p6weekly.wordpress.com/2018/07/30/2018-31-check-out-garbage/
for more details.

Dockerfile

@@ -3,7 +3,7 @@ MAINTAINER Rob Hoelz
 
 RUN groupadd -r perl6 && useradd -r -g perl6 perl6
 
-ARG rakudo_version=2018.04
+ARG rakudo_version=2018.06
 ENV rakudo_version=${rakudo_version}
 
 RUN buildDeps=' \