Bump MSRV to 1.50.0

rustls uses bool::then, for example, stabilized in 1.50:
rust-lang/rust#79299

Note that their real MSRV is 1.52.1 so this may need to be bumped
further.

Author: djc

.github/workflows/rust.yml

@@ -17,7 +17,7 @@ jobs:
           - stable
           - beta
           - nightly
-          - 1.39.0
+          - 1.50.0
         os: [ubuntu-18.04]
         # but only stable on macos/windows (slower platforms)
         include: