Switch to Ubuntu 18.04 (Bionic) for Travis CI build

This patch updates the Travis CI build configuration to use Ubuntu
18.04 (Bionic).

Since Ubuntu 18.04 no longer has separate packages for the different
Apache Multi-Processing Modules, we no longer need to build with both
apache2-prefork-dev and apache2-threaded-dev. We simply need to build
against apache2-dev.

.travis.yml

@@ -1,9 +1,6 @@
-dist: trusty
+dist: bionic
 language: c
-env:
-  - APACHE_PKG=apache2-prefork-dev
-  - APACHE_PKG=apache2-threaded-dev
 install:
   - sudo apt-get update -qq
-  - sudo apt-get install -qq "$APACHE_PKG" libglib2.0-dev liblasso3-dev libssl-dev
+  - sudo apt-get install -qq apache2-dev libglib2.0-dev liblasso3-dev libssl-dev
 script: ./autogen.sh && ./configure CFLAGS=-Werror && make && make distfile