Add support for Rails 5.1 and address deprecations coming with Rails 6

[elrayle]

Maintain support for Rails 5.0; Add support for Rails >= 5.1:

• method index_name_exists?(table_name, index_name, default) had the default parameter removed in Rails 5.1, but it is required in Rails 5.0. There is a check for the Rails version and it passes the default parameter when Rails is < 5.1 and doesn't when >= 5.1

Address deprecations coming with Rails 6:

• be_success is deprecated for Rails 6 and is being replaced with be_successful

[jrochkind]

Maintain support for Rails 4; Add support for Rails >= 5.1:

I don't think this can be maintaining support for Rails 4, when the gemspec already said '~> 5.0'?

As I read it, master already did not support Rails 4, with or without this PR.

It looks like that was done in this commit: https://github.com/samvera/questioning_authority/commit/00eecbdbdc910d72e66fb57448915cc966803386#diff-9c35d0ccb2bff3b9506091c7ed9b89f5

Which makes it look like Rails4 support was actually dropped in qa 2.0.0. I think.

But at this point, with or without this PR, any release off of master will not have support for Rails 4. I think. Unless the gemspec is altered to allow it again (plus it actually works with Rails 4). Am I missing something?

[elrayle]

The description was off. Updated to reflect Rails 5 support only.

[elrayle]

Rails 4 support was removed in this commit... https://github.com/samvera/questioning_authority/commit/00eecbdbdc910d72e66fb57448915cc966803386#diff-9c35d0ccb2bff3b9506091c7ed9b89f5