travis: correct nightly for all steps

.travis.yml

@@ -38,8 +38,8 @@ before_script:
 script:
   # we set release as build type for all steps to benefit from already compiled packages in prior steps
   # skipping clippy ...
-  - cargo +nightly clippy --release --target=${TARGET} -- -D warnings
-  - cargo +nightly test --release --verbose --all --target=${TARGET}
+  - cargo +nightly-2020-05-22 clippy --release --target=${TARGET} -- -D warnings
+  - cargo +nightly-2020-05-22 test --release --verbose --all --target=${TARGET}
   - WASM_BUILD_TOOLCHAIN=nightly-2020-05-22 cargo +nightly build --release --target=${TARGET}
 
 before_deploy: