Support for rails 5.2

[lancecarlson]

I couldn't figure out why it bundler was complaining about this gem using rails 5.2.3. Perhaps it just me?

Bundler could not find compatible versions for gem "activesupport":
  In Gemfile:
    activerecord (~> 5.2.3) was resolved to 5.2.3, which depends on
      activesupport (= 5.2.3)

    standalone_migrations (~> 5.2.7) was resolved to 5.2.7, which depends on
      railties (>= 4.2.7, < 5.3.0) was resolved to 4.2.8.rc1, which depends on
        activesupport (= 4.2.8.rc1)

[jrochkind]

I think bundler is just getting confused, and some variety of bundle update should fix it.

The gemspec say should support anything less than Rails 5.3.