Travis-CI: use "container-based infrastructure"

See <https://docs.travis-ci.com/user/migrating-from-legacy/> for
details.

.travis.yml

@@ -1,7 +1,15 @@
 language: c
-before_install:
+sudo: false
-  - sudo apt-get update -qq
+addons:
-  - sudo apt-get install -qq libident-dev libpam0g-dev libssl-dev libwrap0-dev zlib1g-dev expect telnet
+  apt:
+    packages:
+    - libident-dev
+    - libpam0g-dev
+    - libssl-dev
+    - libwrap0-dev
+    - zlib1g-dev
+    - expect
+    - telnet
 compiler:
   - gcc
   - clang