bump CI Rakudo baseline a little more for deps

dwarring · dwarring · commit e4e524f7e0f5 · 2019-01-19T06:47:38.000+13:00
diff --git a/.travis.yml b/.travis.yml
@@ -4,7 +4,7 @@ language: perl6
 
 perl6:
     - latest
-    - '2018.05'
+    - '2018.06'
 
 install:
     - rakudobrew build zef
