Move tarpaulin before cache

Author: Zibi Braniecki

.travis.yml

@@ -16,7 +16,7 @@ script:
   - cargo test
   - cargo doc
 
-after_success: |
+before_cache: |
   if [[ "$TRAVIS_RUST_VERSION" == stable ]]; then
     RUSTFLAGS="--cfg procmacro2_semver_exempt" cargo install cargo-tarpaulin
     cargo tarpaulin --ciserver travis-ci --coveralls $TRAVIS_JOB_ID