Update Dotenv dependency to use >= as opposed to ~>

[saurabhbhatia]

Hey there,

Thanks for all the work. I am embarking on a fresh integration for Getstream into our Rails app with activity feeds. One of the issues I was facing was a dependency conflict between dotenv-rails and get stream-ruby.

Bundler could not find compatible versions for gem "dotenv":
  In snapshot (Gemfile.lock):
    dotenv (>= 3.1.8)

  In Gemfile:
    dotenv-rails was resolved to 3.2.0, which depends on
      dotenv (= 3.2.0)

    getstream-ruby was resolved to 1.1.1, which depends on
      dotenv (~> 2.0)

Running `bundle update` will rebuild your snapshot from scratch, using only
the gems in your Gemfile, which may resolve the conflict.

In order to keep up with the dotenv-rails versions, I've updated the gemspec dependency to >=.