Travis CI: Switch to installing packages using the APT addon

Now that we no longer need to conditionally install a package, we can
use the APT addon for Travis CI to install the required packages.

See:

* https://docs.travis-ci.com/user/installing-dependencies/#installing-packages-with-the-apt-addon
  (Installing Packages with the APT Addon)

.travis.yml

@@ -1,6 +1,11 @@
 dist: bionic
 language: c
-install:
-  - sudo apt-get update -qq
-  - sudo apt-get install -qq apache2-dev libglib2.0-dev liblasso3-dev libssl-dev
+addons:
+  apt:
+    packages:
+      - apache2-dev
+      - libglib2.0-dev
+      - liblasso3-dev
+      - libssl-dev
+
 script: ./autogen.sh && ./configure CFLAGS=-Werror && make && make distfile