TravisCI: modify to run builds in container

.travis.yml

@@ -1,23 +1,32 @@
 language: c
+
 compiler:
   - gcc
 
-script: 
+env:
-  - autoconf && autoheader && ./configure $BUNDLEDLIBTOM CFLAGS="-O2 -Wall -Wno-pointer-sign $WEXTRAFLAGS" --prefix=$HOME/inst 
+  - BUNDLEDLIBTOM=--disable-bundled-libtom WEXTRAFLAGS=-Werror
+  - BUNDLEDLIBTOM=--enable-bundled-libtom
+  - MULTI=1
+  - NOWRITEV=1
+
+# container-based builds
+sudo: false
+addons:
+  apt:
+    packages:
+    # packages list: https://github.com/travis-ci/apt-package-whitelist/blob/master/ubuntu-precise
+    - zlib1g-dev
+    - libtomcrypt-dev
+    - libtommath-dev
+
+script:
+  - autoconf && autoheader && ./configure $BUNDLEDLIBTOM CFLAGS="-O2 -Wall -Wno-pointer-sign $WEXTRAFLAGS" --prefix=$HOME/inst
   - test "$NOWRITEV" && sed -i s/HAVE_WRITEV/DONT_HAVE_WRITEV/ config.h || true
   - make install
+
+after_success:
   - ~/inst/bin/dropbearkey -t rsa -f testrsa
   - ~/inst/bin/dropbearkey -t dss -f testdss
   - ~/inst/bin/dropbearkey -t ecdsa -f testec256 -s 256
   - ~/inst/bin/dropbearkey -t ecdsa -f testec384 -s 384
   - ~/inst/bin/dropbearkey -t ecdsa -f testec521 -s 521
-
-before_install:
-  - sudo apt-get update -qq
-  - sudo apt-get install -qq libz-dev libtomcrypt-dev libtommath-dev
-
-env: 
-  - BUNDLEDLIBTOM=--disable-bundled-libtom WEXTRAFLAGS=-Werror
-  - BUNDLEDLIBTOM=--enable-bundled-libtom
-  - MULTI=1
-  - NOWRITEV=1