add clang to travis

I think testing mruby with another compiler (the builds are parallel) and also using its undefined behavior sanitizer can be helpful to find programming mistakes. I've disabled the alignment one for now, because of errors in `src/codegen.c`. And since `mrbc` is used a lot during building mruby, the log would contain too much errors (in my opinion). Of course, the alignment issues should be fixed someday.

Example: https://travis-ci.org/cremno/mruby/jobs/24410033#L747-L748