Build with Go 1.15

[sepetrov]

Changed

• CI image to circleci/golang:1.15
• CI machine to ubuntu-1604:202007-01
• Python test to use runtime3.7 instead of runtime2.7

Removed

• vendor directory
• dead code for test using runtime2.7

TODOs:

• [x] Ready to review
• [x] Automated Tests
• [ ] Docs

[andresmgot]

Merging this :+1: