Signal receive send (and unrelated travis-ci)

Hello,

PR content:

1. After seeing #55 I released that `socket::receive` for `zmqpp::signal` was wrong.
This patch update the `send` and `receive` signal overload for `zmqpp::socket`. They now take a `bool dont_block` as second parameter. This makes more sense because any flag expect `DONT_WAIT` doesn't really make sense when sending a signal (I believe). 
2. Adds a test that show a working signal send/receive with blocking / non blocking behavior
3. Update the travis-ci configuration so that it builds. One test is disabled on the travis-ci build.

#56 suggested that the `send` / `receive` methods should be more consistent. I agree, however I didn't change the `std::string` overload because i'm not sure how it would affect other user.