testing on newer gccs and macosx

this patch adds building on ubuntu bionic that has a newer gcc while
keeping ubuntu xenial (default on travis-ci)

it also adds macosx since it was very easy to add and it's a platform we can build for

.travis.yml

@@ -1,3 +1,9 @@
+jobs:
+  include:
+    - dist: xenial
+    - dist: bionic
+    - os: osx
+
 language: c
 
 script: