Revert "Add 5.20 to Travis CI builds" as it looks like Travis has issues with 5.20

This reverts commit 4522b7f31b7e753f4e19af60e55633d7c3e0ee52.

.travis.yml

@@ -5,7 +5,6 @@ perl:
   - "5.12"
   - "5.14"
   - "5.18"
-  - "5.20"
 branches:
   only:
     - master