Move require for omniauth-oauth2 gem to omniauth-msunet.rb file

lib/omniauth-msunet.rb

-require "omniauth-msunet/version"
+require 'omniauth-oauth2'
+require 'omniauth-msunet/version'
 require 'omniauth/strategies/msunet'

lib/omniauth/strategies/msunet.rb

-require 'omniauth-oauth2'
-
 module OmniAuth
   module Strategies
     class Msunet < OmniAuth::Strategies::OAuth2