Clean up Travis ruby version

[janz93]

Remove some of the old ruby versions from the Travis CI setup.

[janz93]

@arbox What are the minimal wanted version which need to be support by the gem? I personally would start by 2.0 but not under it.

[arbox]

I'd say we'll support the minimum version our dependencies support. The code of wlapi itself runs on every Ruby, but savon has other requrements. I'll do it in sync with the dependencies.

[janz93]

Okay great. Also take a look at this failed build. https://ci.appveyor.com/project/arbox/wlapi/build/1.0.466

[arbox]

Then we can put our minimal dependency at 2.2.