ci: Turn of PR builds on Travis
ci: Turn of PR builds on Travis

This commit turns off PR builds happening on Travis, instead entirely
relying on Azure for PR builds to succeed.

aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust