Bump ruby versions to 2.4.2, 2.3.5 and 2.2.8 at Travis
Bump ruby versions to 2.4.2, 2.3.5 and 2.2.8 at Travis

https://www.ruby-lang.org/en/news/2017/09/14/ruby-2-4-2-released/
https://www.ruby-lang.org/en/news/2017/09/14/ruby-2-3-5-released/
https://www.ruby-lang.org/en/news/2017/09/14/ruby-2-2-8-released/

rshell/oracle-enhanced,yahonda/oracle-enhanced,rsim/oracle-enhanced,cdinger/oracle-enhanced,cdinger/oracle-enhanced,rsim/oracle-enhanced,rshell/oracle-enhanced,yahonda/oracle-enhanced