travis: re-enable caching now that build is working

.travis.yml

@@ -38,7 +38,6 @@ before_script:
 script:
   # we set release as build type for all steps to benefit from already compiled packages in prior steps
   # skipping clippy ...
-  - cargo clean
   - BUILD_DUMMY_WASM_BINARY=1 cargo clippy --release --target=${TARGET} -- -D warnings
   - BUILD_DUMMY_WASM_BINARY=1 cargo test --release --verbose --all --target=${TARGET}
   - BUILD_DUMMY_WASM_BINARY=0 WASM_BUILD_TOOLCHAIN=nightly-2020-05-23 cargo build --release --target=${TARGET} -p joystream-node