CI: Add Ruby 2.6.0 to matrix

[olleolleolle]

This PR adds latest MRI to the matrix.

• remove 2.6.0-rc1

[pitr-ch]

Thanks.

     - name: MRI 2.5.1
       rvm: 2.5.1	     

The above should be added so we keep testing all minor versions.

[pitr-ch]

Do you know why the 1.9.3 is running so old bundler? 1.17.3 bundler still supports 1.9.3.

[olleolleolle]

@pitr-ch We can add a before_install to 1.9.3's matrix item, and do a gem install bundler there.

[olleolleolle]

@pitr-ch I added 2.5, and also a before_install for 1.9.3.

[pitr-ch]

Thanks, that works. I did one more cosmetic change, then when it's green I'll merge it.