cpp-netlib 0.11.1RC2 (new formula)

Modified from seemingly abandoned PR https://github.com/Homebrew/homebrew/pull/29511
- upgraded to 0.11.1RC2
- added a test
Not there yet, and the latest stable doesn't build on the current boost 1.56, so if you require stable we'll have to wait until cpp-netlib 0.11.1 is out.