.travis.yml: Bump polyval MSRV to 1.32.0

The POLYVAL tests are failing on Rust 1.27.0, but passing on newer
versions, suggesting there may be something amiss with `core::arch`
support for `pclmulqdq` on older Rust versions.

This bumps it up to our ideal MSRV, as 1.32.0+ would allow us to replace
the dependency on the `byteorder` crate with equivalent `core`
functions.

.travis.yml

@@ -26,8 +26,8 @@ matrix:
         - cargo build --verbose --all --exclude=polyval --tests
 
     # polyval presently needs either RUSTFLAGS or non-default features
-    - name: "Rust: 1.27.0 (polyval)"
-      rust: 1.27.0
+    - name: "Rust: 1.32.0 (polyval)"
+      rust: 1.32.0
       script: ./test_polyval.sh
     - name: "Rust: stable (polyval)"
       rust: stable