travis: move to container-based builds

.travis.yml

@@ -2,10 +2,15 @@ language: cpp
 compiler:
   - gcc
   - clang
-before_install:
+addons:
-  - sudo apt-get update
+  apt:
-install:
+    packages:
-  - sudo apt-get install --no-install-recommends bc doxygen expect gettext libncurses5-dev
+      - bc
+      - doxygen
+      - expect
+      - gettext
+      - libncurses5-dev
+sudo: false
 script:
   - autoreconf
   - ./configure