OAuth flow fails if the version of omniauth-oauth2 gem installed is 1.4.0 or above

[nisanthchunduru]

The omniauth-oauth2 gem (which omniauth-pipedrive depends upon) contains this change omniauth/omniauth-oauth2#70 since version 1.4.0. The change causes the OAuth flow to fail

Ensure the gem is compatible with omniauth-oauth2 1.4.0 and above.

[nisanthchunduru]

Fixed in #4