travis: use official Rust Docker image

.travis.yml

@@ -5,8 +5,8 @@ matrix:
       sudo: required
       dist: trusty
       services: docker
-      install: docker pull ubuntu:18.04
-      script: docker run -v $(pwd):/liftinstall ubuntu:18.04 /bin/bash -ex /liftinstall/.travis/build.sh
+      install: docker pull rust:1
+      script: docker run -v $(pwd):/liftinstall rust:1 /bin/bash -ex /liftinstall/.travis/build.sh
 
     - os: osx
       language: rust

.travis/build.sh

@@ -4,7 +4,4 @@ cd /liftinstall || exit 1
 apt-get update
 apt-get install -y libwebkit2gtk-4.0-dev libssl-dev
 
-curl https://build.travis-ci.org/files/rustup-init.sh -sSf | sh -s -- -y
-export PATH=~/.cargo/bin:$PATH
-
 cargo build