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.
rename stack:migrate to stack:set
simplify stack:set
1.8 compat. fix #1121
enable CI tests on 1.8.7 again

see #1100
fix stack spec
fix ps spec on 1.8.7, no need to generate uuids here
v3.8.2
Merge pull request #1120 from voxik/webmock-1.11.0+-compatibility

WebMock 1.11.0+ compatibility fix.