Use the latest Rubies on Travis CI

[koic]

These Rubies has been released.

• https://www.ruby-lang.org/en/news/2018/03/28/ruby-2-2-10-released/
• https://www.ruby-lang.org/en/news/2018/10/17/ruby-2-3-8-released/
• https://www.ruby-lang.org/en/news/2018/10/17/ruby-2-4-5-released/
• https://www.ruby-lang.org/en/news/2018/10/18/ruby-2-5-3-released/
• https://www.ruby-lang.org/en/news/2018/12/25/ruby-2-6-0-released/
• https://www.jruby.org/2018/12/06/jruby-9-2-5-0

And This PR removes gem update --system from .travis.yml.

RubyGems 3.0.1 requires Ruby 2.3.0 or higher. https://github.com/rubygems/rubygems/blob/v3.0.1/rubygems-update.gemspec#L32

It prevents the following build error.

$ gem update --system
Updating rubygems-update
Fetching: rubygems-update-3.0.1.gem (100%)
ERROR:  Error installing rubygems-update:
        rubygems-update requires Ruby version >= 2.3.0.
ERROR:  While executing gem ... (NoMethodError)
    undefined method `version' for nil:NilClass
The command "gem update --system" failed and exited with 1 during .

[nashby]

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