WebMock 1.11.0+ compatibility fix.

Excon mocking is disable by WebMock::HttpLibAdapters::ExconAdapter.disable!
call since WebMock 1.11.0 by
https://github.com/bblimke/webmock/commit/3639da1a1c8bbc067a546dc2708c5a759de9b660
commit.

Not sure if this is the best fix, but it works for me. May be it should be improved on WebMock side instead?