Set up CI with Azure Pipelines (#926)

Use Azure Pipelines for CI. This migrates away from Travis and
Appveyor.

tokio-tls/tests/smoke.rs

@@ -364,7 +364,7 @@ description should mention "tokio-tls".
         }
 
         fn local_root_store() -> CertStore {
-            if env::var("APPVEYOR").is_ok() {
+            if env::var("CI").is_ok() {
                 CertStore::open_local_machine("Root").unwrap()
             } else {
                 CertStore::open_current_user("Root").unwrap()