CI: Bump minimum supported Rust version to 1.44.1

for the protobuf crate

.appveyor.yml

@@ -16,7 +16,7 @@ environment:
       target: x86_64-pc-windows-msvc
     - channel: stable
       target: i686-pc-windows-msvc
-    - channel: 1.36.0
+    - channel: 1.44.1
       target: x86_64-pc-windows-msvc
 
 install:

.travis.yml

@@ -4,7 +4,7 @@ rust:
   - stable
   - beta
   - nightly
-  - 1.36.0
+  - 1.44.1
 script:
   - |
       cargo test --verbose;