Drop older Ruby and Rails support

[scarroll32]

Bring Ransack into line with Polyamorous and drop some older versions.

https://github.com/activerecord-hackery/polyamorous/blob/master/.travis.yml

Posted here in case before proceeding in case this breaks other code, eg ActiveAdmin activeadmin/activeadmin#5254 cc @varyonic

[seanlinsley]

I'm always a fan of reducing the complexity of libraries by only supporting the latest major versions of co-dependencies; it's not worth the extra burden it puts on maintainers. If people on old Rails versions want a new feature, they can patch it into their own fork or convince a maintainer to backport it.

Though I would recommend an X or Y version bump (of X.Y.Z) so that it's easy to release bug fixes to older versions if necessary.

[varyonic]

Ruby 2.2 is EOL end of this March, 2018.
Rails 4.2 will receive severe security fixes until 6.0 is available.
It would be nice to match these dates for library maintenance, but not required if it is indeed a burden.

[avit]

+1 on this to simplify everything. I had suggested moving the multiple version support around in #416 but never got back around to working on it.

I think it could be structured it so the current supported version is the explicit one, but there can still be some facility to include backports as separate modules if anyone wants to contribute them.

Personally I still depend on ransack with a Rails 3.2 application (I know, ouch), but can't expect the community to continue supporting that!

[scarroll32]

@varyonic that makes sense.

[scarroll32]

So do we go forward with the below?

Rails

5.1, 5.0 and 4.2

As per this page

Ruby

2.2 until end of March 2018
2.3
2.4

As per this page