travis: add linux, osx, clang and gcc targets

Signed-off-by: Tristan Matthews <tmatth@videolan.org>
diff --git a/.travis.yml b/.travis.yml
index 43a3b1a..57376a7 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -15,3 +15,11 @@
   - if [[ "$BUILD" == "CMAKE" ]] ; then cmake --build . ; fi
   - if [[ "$BUILD" == "CMAKE" ]] ; then cmake --build . --target install ; fi
   - if [[ "$BUILD" == "CMAKE" ]] ; then popd ; fi
+
+os:
+- linux
+- osx
+
+compiler:
+- gcc
+- clang