Fix equations in classifier/NHERD update

This PR fixes [#816](https://github.com/jubatus/jubatus/issues/816).
NHERD update equations were wrong. It must have ``variance`` in the dominators instead of ``val_covariance_xxx`` in [L89](https://github.com/jubatus/jubatus_core/blob/master/jubatus/core/classifier/normal_herd.cpp#L89) and [L100](https://github.com/jubatus/jubatus_core/blob/master/jubatus/core/classifier/normal_herd.cpp#L100).
It also appends hinge loss function ``std::max(0.f, 1.f - margin)`` as in the original paper for safety.
For large ``regularization-weight`` such as 100, NHERD is now stable and no ``Inf`` appeared.
`std::max` is not required here because `margin` is always lesser then `1.f` https://github.com/jubatus/jubatus_core/pull/47/files#diff-179ba8ad54c29221ca44d6816d611cd8L56