Fix/130 ignore socket errors

[luismedel]

This PR adds a way to ignore the dreaded socket.gaierror error when initializing the UDP client and a server is missing.

• Fixes socket.gaierror is raised if the host is not known #130
• (Hopefully) fixes Add a method to re-run DNS lookup #125

Details

The new constructor signature is:

StatsClient(host='localhost', port=8125, prefix=None, maxudpsize=512, ignore_socket_errors=False)

To complement this flag, I added the is_ready property and the reset() method to clients.

On the UDP client:

• By default, the client will work as usual. When passing ignore_socket_errors=True, errors during the socket initialization will be ignored.
• The is_ready property will reflect the state after the initialization.
• In order to be able to recover from a failed initialization you can use the reset() method.

On streaming clients:

• They will work as usual (won't ignore errors)
• is_ready reflects the current status of the inner socket.
• reset() calls reconnect() under the hood.

Extra

I added a quick Travis detection to skip the tests dynamically.

[luismedel]

Trying to ignore Travis :-)