Prevent passing of non-POD types to POD-only interfaces.

Reviews: @rec @howardhinnant
He's using `std::enable_if_t` from C++14.  :-)

On Fri, Dec 19, 2014 at 5:02 PM, Donovan Hide <notifications@github.com>
wrote:
>
> In src/ripple/crypto/RandomNumbers.h
> <https://github.com/ripple/rippled/pull/761#discussion-diff-22131892>:
>
> > @@ -21,6 +21,7 @@
> >  #define RIPPLE_CRYPTO_RANDOMNUMBERS_H_INCLUDED
> >
> >  #include <beast/utility/Journal.h>
> > +#include <beast/cxx14/type_traits.h> // <type_traits>
>
> Are std::enable_if and std::is_pod not part of C++11 and in ?
> http://en.cppreference.com/w/cpp/types/enable_if
> http://en.cppreference.com/w/cpp/types/is_pod
>
> —
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> <https://github.com/ripple/rippled/pull/761/files#r22131892>.
>