Add .travis.yml

Retrieved from https://github.com/hvr/multi-ghc-travis and slightly
edited.

Fixes #5
Use `Data.ByteString.Lazy.Builder` for HP-compatibility

Fixes #1
Fix `Command` `Show` instance

Some versions of `Builder` don't have a `Show` instance, so a manual
instance is required.

This one was generated by GHC's `-ddump-deriv` option, Vim's
search-and-replace function, and lots of patience.

Fixes #4