dgtal 0.8.0

DGtal upgrade to 0.8.0 and formula cleanup
In that case, two more changes:

1. `needs :cxx11` above `def install` to blacklist compilers that don't work with C++11
1. something like `boost_args = []; boost_args << "c++11" if MacOS.version < "10.9"; depends_on "boost" => boost_args`