install and use gnu-grep for testing in Darwin

MacOSX uses BSD grep instea of GNU grep, which lacks the -P argument
used by the `_includes()` test.  Therefore, for now let's just install
it on travis.

Homebrew installs as "ggrep", which we will *not* redefine, since we
only want to use it here in the texts explicitly, but don't want to
change the environment used by the OS for most things (because some of
the shell scripts call grep, and we want to test behavior on a normal
OSX system)

Ideally we would find a replacement for the grep command here entirely
that doesn't rely on this, so that tests can work on any MacOSX system
without extra dependencies, but let's approach that later.