Switch Windows CI builds from windows-gnu to windows-msvc.

The Travis default of windows-gnu suffers from rust-lang/rust#47048, I
think. I don't see an easy way to address this right now, but switching
our Windows builds to windows-msvc will avoid this and is more realistic
target anyway.

.travis.yml

@@ -13,11 +13,15 @@ env:
     - RELEASE=
     - RELEASE=--release
 
+install:
+  - if [ "$TRAVIS_OS_NAME" = "windows" ]; then rustup target add x86_64-pc-windows-msvc; fi
+
 script:
   # We cannot run `cargo test` with the mp4parse_fallible feature enabled
   # (see comment where the feature is defined in mp4parse_capi/Cargo.toml),
   # but we can at least check for changes behind features that would break the
   # build.
   - cargo check --all --verbose $RELEASE --tests --all-features
-  - cargo test --all --verbose $RELEASE
+  - if [ "$TRAVIS_OS_NAME" = "windows" ]; then export TARGET=--target=x86_64-pc-windows-msvc; fi
+  - cargo test --all --verbose $RELEASE $TARGET
   - cargo doc --package mp4parse_capi