.travis.yml: fix dpkg lock issue, hopefully; try to use containers

.travis.yml

@@ -5,18 +5,20 @@
 language: rust
 
 cache: cargo
-sudo: true
 
 matrix:
   include:
     - os: linux
+      sudo: required
       dist: xenial
       rust: stable
-      env: RUSTFLAGS="-C link-dead-code"
+      env: RUSTFLAGS="-C link-dead-code" SUDO=sudo
     - os: linux
+      sudo: false
       dist: xenial
       rust: beta
     - os: linux
+      sudo: false
       dist: xenial
       rust: nightly
     - os: osx
@@ -47,13 +49,15 @@ before_install:
     if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
       brew update
     else
-      sudo systemctl stop apt-daily.service &&
-      sudo systemctl kill --kill-who=all apt-daily.service &&
-      while ! (systemctl list-units --all apt-daily.service | fgrep -q dead) ; do
-        sleep 1
-      done &&
-      sudo add-apt-repository -y ppa:k-peter/tectonic-ci &&
-      sudo apt-get update
+      if [[ "$TRAVIS_RUST_VERSION" == "stable" ]]; then
+        sudo systemctl stop apt-daily.service &&
+        sudo systemctl kill --kill-who=all apt-daily.service &&
+        while ! (systemctl list-units --all apt-daily.service | fgrep -q dead) ; do
+          sleep 1
+        done
+      fi &&
+      $SUDO add-apt-repository -y ppa:k-peter/tectonic-ci &&
+      $SUDO apt-get update
     fi
 
 install:
@@ -63,7 +67,7 @@ install:
       brew install harfbuzz --with-graphite2 &&
       brew install --force openssl
     else
-      sudo apt-get install -y libharfbuzz-dev
+      $SUDO apt-get install -y libharfbuzz-dev
     fi
 
 before_script: