Fix broken error handling specs

The reason the specs are failing is because of the following change
in the webmock gem:
bblimke/webmock@1c48030#diff-4fd2506a42939f27882b390651103483

RequestStub:to_timeout method now raises a Net::OpenTimeout exception
instead of a Timeout::Error. The Net::OpenTimeout error in the
NetHttp adapter is wrapped in a Faraday::ConnectionFailed error.

spec/support/shared_examples/error_handling.rb

@@ -93,7 +93,7 @@
   end
 
   describe 'response timeout' do
-    let(:response) { request_stub.to_timeout }
+    let(:response) { request_stub.to_raise(Timeout::Error) }
 
     it 'raises Skroutz::TimeoutError' do
       expect { request }.to raise_error(Skroutz::TimeoutError)