add sample recordings submodule

git submodules are usually considered harmful but in this case they are
useful precisely because the recordings module is not needed to actually
build the code. It is useful to document the location of a set of
recordings however.
README: add note on recordings submodule
travis: disable submodules

Don't have travis check our the recording submodule for the moment.
Disable it until we need it.
Merge pull request #6 from rjw57/add-recordings-submodule

add sample recordings submodule