Adding libunwind8 to .travis.yml

[skip appveyor]

Author: Nate McMaster

.travis.yml

@@ -10,6 +10,10 @@ os:
   - linux
   - osx
 osx_image: xcode8.2
+addons:
+  apt:
+    packages:
+      - libunwind8
 branches:
   only:
     - master