Fix rubocop Ruby 1.9.3 compatibility issues

lib/googleauth/user_refresh.rb

@@ -75,7 +75,7 @@ module Google
       # JSON key.
       def self.read_json_key(json_key_io)
         json_key = MultiJson.load(json_key_io.read)
-        wanted = %w[client_id client_secret refresh_token]
+        wanted = %w(client_id client_secret refresh_token)
         wanted.each do |key|
           raise "the json is missing the #{key} field" unless json_key.key?(key)
         end

lib/googleauth/web_user_authorizer.rb

@@ -227,7 +227,7 @@ module Google
       # @param [Rack::Request] request
       #  Current request
       def self.validate_callback_state(state, request)
-        if state[AUTH_CODE_KEY].nil? # rubocop:disable Style/GuardClause
+        if state[AUTH_CODE_KEY].nil?
           raise Signet::AuthorizationError, MISSING_AUTH_CODE_ERROR
         elsif state[ERROR_CODE_KEY]
           raise Signet::AuthorizationError,