Avoid Ruby 2.1.0 crash due to Excon bug when connection is failed

geemus/excon#358

test/adapters/excon_test.rb

@@ -12,6 +12,9 @@ module Adapters
       # Excon lets OpenSSL::SSL::SSLError be raised without any way to
       # distinguish whether it happened because of a 407 proxy response
       undef :test_proxy_auth_fail if ssl_mode?
+
+      # https://github.com/geemus/excon/issues/358
+      undef :test_connection_error if RUBY_VERSION >= '2.1.0'
     end
   end
 end