Travis CI: lock nightly to a version with Clippy

Signed-off-by: Nick Cameron <nrc@ncameron.org>

Author: nrc

.travis.yml

@@ -28,6 +28,9 @@ addons:
       - go
 
 install:
+  # FIXME(#55) Remove when Clippy is fixed in current nightly
+  - if [[ $TRAVIS_OS_NAME == "linux" ]]; then rustup toolchain install nightly-2019-05-22; fi
+  - if [[ $TRAVIS_OS_NAME == "linux" ]]; then rustup default nightly-2019-05-22; fi
   - if [[ $TRAVIS_OS_NAME == "linux" ]]; then rustup component add rustfmt; fi
   - if [[ $TRAVIS_OS_NAME == "linux" ]]; then rustup component add clippy; fi
   - if [[ $TRAVIS_OS_NAME == "windows" ]]; then choco install golang cmake strawberryperl protoc; fi