Relax activesupport version constraint

[mlarraz]

Right now this prevents installing any patch releases of Rails 5.2.

Given the Gemfile:

source 'https://rubygems.org'

gem 'flip', github: 'pda/flip'
gem 'activesupport', '~> 5.2.3'

Running bundle install gives:

Bundler could not find compatible versions for gem "activesupport":
  In Gemfile:
    activesupport (~> 5.2.3)

    flip was resolved to 1.1.1, which depends on
      activesupport (>= 4.0, <= 5.2)