Fix tarpaulin maybe

Author: Zibi Braniecki

.travis.yml

@@ -17,7 +17,7 @@ script:
 - cargo test
 
 before_cache: |
-  if [[ "$TRAVIS_RUST_VERSION" == stable ]]; then
+  if [[ "$TRAVIS_RUST_VERSION" == nightly ]]; then
     RUSTFLAGS="--cfg procmacro2_semver_exempt" cargo install cargo-tarpaulin
-    cargo tarpaulin --ciserver travis-ci --coveralls $TRAVIS_JOB_ID
+    cargo +nightly tarpaulin --ciserver travis-ci --coveralls $TRAVIS_JOB_ID
   fi