p2p/net/swarm: fix travis env flag val
p2p/net/swarm: fix travis env flag val

the travis docs say the env flag TRAVIS=true but, i want
to make absolutely sure. explicitness leaves less room for
error.

zippy/go-libp2p,ipfs/go-libp2p,libp2p/go-libp2p,libp2p/go-libp2p