add Flang (PGI one) support in Travis

[jeffhammond]

PGI has released Flang based on the PGI front-end. We know that the PRKs work with this compiler, thanks to #153 and #154.

The only challenge here is building the full Flang toolchain in a Travis instance, given how long it takes. It may be prudent instead to support PGI Community Edition for now (see #139) and wait until Flang is merge into LLVM or at least has a Homebrew port.