Update the ruby version to 2.6.1

[seanmarcia]

This PR does three things:

• It updates the ruby version from 2.5.3 to 2.6.1
• It refactors our filterable concern to change the method name from filter to class_filter. This is because with the release of ruby 2.6.0 they created filter as an alias on select on enumerable. I'm not entirely sure I like the name class_filter but I couldn't think of anything better at the time.
• Updates our capistrano deploy scripts to use ruby 2.6.1

[armahillo]

class_filter is fine for now.

I have a gem started for that feature that I want to work on anyways, so eventually it will be something else :)