CI: add cargo cache to build faster

Author: mssun

.travis.yml

@@ -1,4 +1,5 @@
 language: rust
+cache: cargo
 rust:
   - 1.26.2
   - stable
@@ -35,3 +36,6 @@ script:
   - if [[ "$TARGET" =~ "x86_64-unknown-linux-*" ]]; then cargo test --verbose --all --target $TARGET; fi
   - if [[ "$COVERAGE" == "1" ]]; then ./ci/run-cov; fi
   - if [[ "$COVERAGE" == "1" ]]; then bash <(curl -s https://codecov.io/bash) -f final.info; fi
+notifications:
+  email:
+    on_success: never