travis: switch from Ubuntu 14.04 to 16.04

Travis is phasing out its support for containers, so we remove the `sudo:
false`, which will be a no-op soon.

Reference: https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration

Changes for Linux:

  - Xenial comes with libtool installed already. It only provides "libtoolize",
    though. For "libtool" we need to install libtool-bin.

.travis.yml

@@ -1,5 +1,5 @@
-dist: trusty
-sudo: false
+dist: xenial
+
 language: c
 
 env:
@@ -96,9 +96,9 @@ before_cache:   ci/before_cache.sh
 addons:
   apt:
     packages:
+      - apport
       - autoconf
       - automake
-      - apport
       - build-essential
       - clang
       - cmake
@@ -108,7 +108,7 @@ addons:
       - gdb
       - language-pack-tr
       - libc6-dev-i386
-      - libtool
+      - libtool-bin
       - locales
       - ninja-build
       - pkg-config

ci/install.sh

@@ -8,10 +8,8 @@ if [[ "${CI_TARGET}" == lint ]]; then
 fi
 
 if [[ "${TRAVIS_OS_NAME}" == osx ]]; then
-  brew install ninja
-  brew install gettext
-  brew reinstall libtool
   brew install ccache
+  brew install ninja
   export PATH="/usr/local/opt/ccache/libexec:$PATH"
 fi