Update Travis CI base.

Previously we used "trusty" (Ubuntu 14.04).
Now we use "bionic" (Ubuntu 18.04).

Note that the current Travis CI default is "xenial" (Ubunu 16.04), which
we are jumping over.

.travis.yml

@@ -1,5 +1,5 @@
 ##
-## Copyright (c) 2017 The University of Utah
+## Copyright (c) 2017, 2019 The University of Utah
 ## All rights reserved.
 ##
 ## This file is part of `csmith', a random generator of C programs.
@@ -29,7 +29,7 @@
 ###############################################################################
 
 sudo: required
-dist: trusty
+dist: bionic
 language: cpp
 
 # before_install: