travis: rust nightly lock to version 2020-05-22 build fails in newer version

.travis.yml

@@ -22,29 +22,25 @@ matrix:
   # building all platform binanires and wasm blob to a different system.
 
 install:
-  - rustup update nightly
-  - rustup target add wasm32-unknown-unknown --toolchain nightly
+  - rustup update install nightly-2020-05-22
+  - rustup target add wasm32-unknown-unknown --toolchain nightly-2020-05-22
   # travis installs rust using rustup with the "minimal" profile so these tools are not installed by default
   - rustup component add rustfmt
   # choosing to add clippy for the nightly toolchain only
   # becuase we want to run it with nighly to avoid having to set BUILD_DUMMY_WASM_BINARY=0
   # setting that variable requires changing it in a later run of cargo bulid to ensure runtime/build.rs
   # actually builds the runtime .. the behaviour is a bit odd so choosing to avoid messing with it.
-  - rustup component add clippy --toolchain nightly
+  - rustup component add clippy --toolchain nightly-2020-05-22
 
 before_script:
   - cargo fmt --all -- --check
 
 script:
-  # dump the cache
-  - cargo clean
-  - rm -fr ~/.cargo/git
-  - rm -fr ~/.cargo/registry
   # 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=${TARGET} -- -D warnings
-  - cargo +nightly -Z minimal-versions test --release --verbose --all --target=${TARGET}
-  - WASM_BUILD_TOOLCHAIN=nightly cargo +nightly -Z minimal-versions build --release --target=${TARGET}
+  - cargo +nightly clippy --release --target=${TARGET} -- -D warnings
+  - cargo +nightly test --release --verbose --all --target=${TARGET}
+  - WASM_BUILD_TOOLCHAIN=nightly-2020-05-22 cargo +nightly build --release --target=${TARGET}
 
 before_deploy: