travis: Don’t install external version of rust-protobuf anymore

.travis.yml

@@ -6,13 +6,9 @@ addons:
   apt:
     packages:
       - protobuf-compiler
+      - libprotoc-dev
       - portaudio19-dev
-      - libavahi-compat-libdnssd-dev
 
-before_install:
-    - export PATH=$HOME/.cargo/bin:$PATH
-install:
-    - cargo install --git https://github.com/stepancheg/rust-protobuf --rev 921c69b protobuf
 script:
     - cargo build --no-default-features --verbose
     - cargo test --no-default-features --verbose