Add Rails 5.2 to the build matrix

[no-reply]

Rails 5.0.x is no longer supported. Drop it from the build matrix and replace it with 5.2.

@samvera/hyrax-code-reviewers

[cjcolvar]

I'm pretty certain rails 5.2 isn't going to work until active-fedora gets a release that includes samvera/active_fedora#1312. Other dependencies may need patches as well.
I think hydra-pcdm is okay but I believe hydra-works has a couple small issues to resolve. Even then Hyrax will need to upgrade to that version of ActiveFedora: #2256 and the other dependencies.

I guess I'm wondering if we should wait on this until we know that we're closer to a PR adding support for rails 5.2. That way we wouldn't be spending extra travis concurrent builds for something we know is going to fail.

[no-reply]

@cjcolvar yes, it's definitely not my intent that we will change the build matrix until this is passing. This is here mainly to provide a target and a place for discussion.

This PR represents the intent to drop support for Rails 5.0 in a future release. We need to talk about how to approach that drop, what it means for our versions, etc...