Don't force-install tarpaulin; it's slow

We can instead just clear the coverage cache if we want an upgrade.

.travis.yml

@@ -81,7 +81,7 @@ jobs:
       rust: nightly
       env: CACHE_NAME=coverage
       script:
-        - RUSTFLAGS="--cfg procmacro2_semver_exempt" cargo install --force cargo-tarpaulin || true
+        - RUSTFLAGS="--cfg procmacro2_semver_exempt" cargo install cargo-tarpaulin || true
         - cargo tarpaulin --out Xml
         - bash <(curl -s https://codecov.io/bash)
         - cargo clean -p imap # ensure we don't cache build for coverage