Replace boost::reverse_iterator with std::reverse_iterator

[sweemer]

@lballabio Do you remember why this code uses boost::reverse_iterator even though std::reverse_iterator has been around since before C++11? As far as I can tell, they both have the same requirements (see here and here) so I don't think there should have been any benefit of using boost over std originally.

This change affects the public interface, but I think it's fine, because users should be using either auto or the typedef type, not the underlying type.

[coveralls]

Coverage: 71.964% (+0.0004%) from 71.963% when pulling b2fa56a on sweemer:reverse_iterator into 5182331 on lballabio:master.

[lballabio]

I don't remember, but if I had to take I guess I'd say that back in the mesozoic period some of the compilers were slow in implementing everything that was defined in the C++ standard library. Kids have it easy these days 😄