Update modis for Rails 5.2 support

[garettarrowood]

@aried3r - See rpush/modis#19 for modis's upgrade to support Rails 5.2 .

[aried3r]

Hey, thanks for the PR!

Is it necessary to force version 3+ of Modis? 2 should still work for Rails <5.2, right? Maybe we can relax the requirement a bit.

[garettarrowood]

modis 2.1 does work for Rails <5.2. But 3.0 works for all maintained versions of Ruby and Rails.

We (added then) removed CI testing for Rails version <4.2. There did appear to be some issues with later versions of cRuby & jRuby with Rails 4.0 & 4.1. However, I believe those issues were there before. Those combos just were not being tested. In other words, there weren't any backwards compatible breaking changes added to the new release.

The version could be relaxed with a >= 2.0 specification. But I can't think of a benefit of doing so.

[aried3r]

Normally I would disagree and keep compatibility with older versions, but since this gem is basically empty I think it will be ok, since people probably don't depend on modis directly if they use this gem.