Add clang++ to Travis CI configuration

[briansmith]

I think mozilla::pkix requires clang 3.4 or later. Preferably, clang 3.4 and the latest release would be supported. More ideally, we'd have a configuration for clang trunk, but it may be better to do that separately since it would mean building clang from source.

[briansmith]

Fixed in the feature/openssl branch.