travis: add lmdb to build

Hi Mark,

this commit adds installation of lmdb libs + building pdns against them to our Travis run. I recommend enabling Travis for your fork (go to travis-ci.org, log in with Github, flip the switch on mzealey/pdns) so you get the benefits of these tests and can easily extend them. Eventually we'd like to see lmdb pop up in the series of start-test-stop commands in this file :)