Fix compatibility with omniauth-oauth 1.4+

[pjg]

References:

• Don't override callback_url Attempt to correct #28 omniauth/omniauth-oauth2#70
• omniauth/omniauth-oauth2@2615267
• fix broken backward compatibility (callback_url) omniauth/omniauth-oauth2#82
• callback url does not work proberly jdennes/omniauth-createsend#3
• Fix omniauth-oauth2 1.4 callback_url regression WebTheoryLLC/omniauth-twitch#4
• Fix compatibility with omniauth-oauth2 v1.4+ jdennes/omniauth-createsend#5
• Reorganize requires and fix compatibility with omniauth-oauth 1.4+ DripEmail/omniauth-drip#6
• Fix compatibility with omniauth-oauth 1.4+ Kosmin/omniauth-verticalresponse#3

[redtachyons]

@asanghi Please merge this PR, build issue is unrelated

[jagthedrummer]

Would be nice to have this merged and a version bump.

[jagthedrummer]

In the mean time we're doing this in an initializer:

# Constant Contact Monkey Patch ---------------------------------------------
# Once the following PR is merged and a new version of the gem is released we
# can ditch this monkey patch.
# https://github.com/asanghi/omniauth-constantcontact2/pull/8
module ConstantContactMonkeyPatch
  def callback_url
    full_host + script_name + callback_path
  end
end
OmniAuth::Strategies::ConstantContact2.include ConstantContactMonkeyPatch
# End Constant Contact Monkey Patch -----------------------------------------