Remove const workaround for IPAddress

https://github.com/arduino/Arduino/pull/1798 was merged, so this is no
longer needed. This means that this code will only work with a recent
nightly build, the Arduino 1.5.6 release does not have this fix yet.
Replace GSClient with GSTcpClient

This name recently changed in the gainspan library, this changes makes
things work with the most recent version of that library again.