Update build matrix to include OSX, Xenial and Bionic

This commit adds OSX to the build matrix, as well as building on Trusty,
Xenial and Bionic, both with GCC and CLang on all distributions. For
now, the OSX build uses the default image provided by Travis CI, but we
may decide to add more images in the future.

Also move realpath installation out of addons::apt::packages, since on
Xenial and newer distributions, realpath is included in the coreutils
package and doesn't need to be explicitly installed. However, we still
need it on Trusty, so add a line to manually install it.

.travis.yml

@@ -1,20 +1,50 @@
 language: c
 script: ./.travis_build.sh
 
+# Compile on both MacOS and Linux
+os:
+    - osx
+    - linux
+
 # Compile using both gcc & clang
 compiler:
     - gcc
     - clang
 
-# Use container based infrastructure
+jobs:
-sudo: false
+    include:
+        - os: linux
           dist: trusty
+          compiler: gcc
+        - os: linux
+          dist: trusty
+          compiler: clang
+        - os: linux
+          dist: xenial
+          compiler: gcc
+        - os: linux
+          dist: xenial
+          compiler: clang
+        - os: linux
+          dist: bionic
+          compiler: gcc
+        - os: linux
+          dist: bionic
+          compiler: clang
+
+# Install dependencies
 addons:
     apt:
         packages:
         - libusb-1.0-0-dev
         - libudev-dev
-        - realpath
+    homebrew:
+        packages:
+        - libusb
+
+# Ensure realpath is available on Trusty
+before_script:
+    - if [ $TRAVIS_OS_NAME = linux ]; then sudo apt-get install realpath || true; fi
 
 # Enable parallel make
 env: