Add travis-ci configuration

[gmr]

This job fails consistently with Erlang 17.5 for me, but passed for R16B03-1 and 18.0. I don't know if it's a timing issue or what, but the .travis.yml "works".

Here's where 17.5 hangs:

rabbit_federation_exchange_test
-------------------------------
simple:                            [running] [PASSED].
multiple_upstreams:                [running] [PASSED].
multiple_uris:                     [running] [PASSED].
multiple_downstreams:              [running] [PASSED].
e2e:                               [running] [PASSED].
unbind_on_delete:                  [running] [PASSED].
unbind_on_unbind:                  [running] [PASSED].
unbind_gets_transmitted:           [running] [PASSED].
no_loop:                           [running] [PASSED].
dynamic_reconfiguration:           [running]

[michaelklishin]

Yes, very likely timing.