upgrade oauth2 gem

[njt1982]

Make it match https://github.com/kmrshntr/omniauth-slack/blob/v2.3.0/omniauth-slack.gemspec

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From #3

The causes conflicts when trying to use other gems such as slack...

Bundler could not find compatible versions for gem "omniauth-oauth2":
  In snapshot (Gemfile.lock):
    omniauth-oauth2 (= 1.0.2)

  In Gemfile:
    omniauth-harvest was resolved to 0.1.0, which depends on
      omniauth-oauth2 (~> 1.0.0)

    omniauth-slack (~> 2.3) was resolved to 2.3.0, which depends on
      omniauth-oauth2 (~> 1.3.1)

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

Is there something specific about this gem which is incompatible with the 1.3.x branch?

[njt1982]

Using this on my project and I can still oauth2 authenticate and get tokens...