Fix PoolingAllocator build errors with older GCCs (#632)

Fix PoolingAllocator build errors with older GCCs (#632)

    error: no class template named rebind in class PoolingAllocator

GCC v4.8.4 (at least) does not fully implement C++11 Allocator-related
APIs, forcing the developer to manually provide Allocator traits that
are supposed to come automatically via std::allocator_traits.

The problem may only manifest itself when using a custom allocator with
types such as std::map<T> and std::list<T> that allocate wrapper
objects instead of Ts. For example, std::vector is unaffected.