CI: Bump minimum supported Rust version to 1.27.2

.appveyor.yml

     - channel: stable
       target: i686-pc-windows-msvc
       cargoflags: --no-default-features
-    - channel: 1.26.2
+    - channel: 1.27.2
       target: x86_64-pc-windows-msvc
 
 install:

.travis.yml

   - stable
   - beta
   - nightly
-  - 1.26.2
+  - 1.27.2
 script:
   - |
-      if [ "$TRAVIS_RUST_VERSION" == "1.26.2" ]; then
+      if [ "$TRAVIS_RUST_VERSION" == "1.27.2" ]; then
         cargo test --verbose;
         cargo doc --verbose --no-deps;
       else