Bump Ruby versions at Travis CI
Bump Ruby versions at Travis CI

* Ruby 3.0.3 Released
https://www.ruby-lang.org/en/news/2021/11/24/ruby-3-0-3-released/

* Ruby 2.7.5 Released
https://www.ruby-lang.org/en/news/2021/11/24/ruby-2-7-5-released/

`gem update --system` added to workaround this error.
 - `require': superclass mismatch for class StringIO (TypeError)

rsim/oracle-enhanced,yahonda/oracle-enhanced,rsim/oracle-enhanced,yahonda/oracle-enhanced