Update Travis CI: earthly 0.7.2 → 0.8.16, drop obsolete protobuf-compiler install

Author: defstream

.travis.yml

@@ -19,16 +19,13 @@ jobs:
   include:
     - name: standard build
       script:
-        - sudo apt-get update -y
-        - sudo apt-get install -y --no-install-recommends protobuf-compiler
         - curl -sSf https://sh.rustup.rs | sh -s -- --default-toolchain none -y
         - export PATH=$HOME/.cargo/bin:$PATH
-        - rustup default stable
         - cargo check
 
     - name: earthly build
       services:
         - docker
       script:
-        - wget https://github.com/earthly/earthly/releases/download/v0.7.2/earthly-linux-amd64 -O earthly && chmod +x earthly && ./earthly bootstrap
-        - ./earthly -P --ci +archive
+        - wget https://github.com/earthly/earthly/releases/download/v0.8.16/earthly-linux-amd64 -O earthly && chmod +x earthly && ./earthly bootstrap
+        - ./earthly --ci +archive