try to make travis happy

.travis.yml

@@ -16,10 +16,10 @@ before_script:
 script:
   - |
     if [[ "$TRAVIS_RUST_VERSION" == "nightly"* ]]; then cargo fmt --all -- --check; fi
-  - cargo build
+  - cargo clean && cargo build
-  - cargo build --features toml
+  - cargo clean && cargo build --features toml
-  - cargo build --features all
+  - cargo clean && cargo build --features all
-  - cargo test --features all
+  - cargo clean && cargo test --features all
 after_success: |
   if [[ "$TRAVIS_RUST_VERSION" == stable ]]; then
     RUSTFLAGS="--cfg procmacro2_semver_exempt" cargo install cargo-tarpaulin