Remove command gem update from travis build

This were being executed to fix an issue related to rubygems
(rubygems/rubygems#763).
This was fixed and merged 1 year ago.

.travis.yml

@@ -41,6 +41,3 @@ notifications:
     on_success: always
     on_failure: always
     use_notice: false
-
-before_install:
-  - gem update --system #  todo: workaround for https://github.com/rubygems/rubygems/pull/763