Improve Travis coverage.

Our official builds use clang-3.4 (for macOS) and clang-3.8 for Linux.
Linux uses libstdc++-4.9. Make sure these two compilers are being tested
and that libstdc++-4.9 is being used for STL.

Add a macOS builder to get coverage there. This will use a newer
clang than we actually use, but as opposed to the linux builder will
test the platform-specific bits.

Finally, use the latest GCC and clang versions from a bionic image, so
we have some coverage of a popular distribution.

tools/checkout-deps.sh

@@ -136,7 +136,7 @@ if [ $? -eq 1 ]; then
   checkout
 
   cd ambuild
-  if [ $iswin -eq 1 ]; then
+  if [ $iswin -eq 1 ] || [ $ismac -eq 1 ]; then
     python setup.py install
   else
     python setup.py build