Relax dependency on omniauth-oauth2

[ryanwood]

I'm upgrading a Rails app from 4.2.x to 5.0.x and ran into a dependency problem with this gem.

Bundler could not find compatible versions for gem "rack":
  In Gemfile:
    rails (~> 5.0.1) was resolved to 5.0.1, which depends on
      actionpack (= 5.0.1) was resolved to 5.0.1, which depends on
        rack (~> 2.0)

    ...
    omniauth-emma (>= 0.0.2) was resolved to 0.0.2, which depends on
      omniauth-oauth2 (~> 1.1.1) was resolved to 1.1.1, which depends on
        oauth2 (~> 0.8.0) was resolved to 0.8.0, which depends on
          rack (~> 1.2)

It looks like the problem is solved with this commit.

Privy@387b5b2

Would it be possible to merge that into this repo?