GCC 4.6 is_nothrow_move_assignable

I know that C++11 on GCC < 4.7 is awful, but `true` is not a good default if there is not way to determine noexceptness.

```cpp
#include

struct foo
{
foo& operator=(foo&&) noexcept(false) { throw 0; }
};

static_assert(!boost::is_nothrow_move_assignable::value, "");
```

https://godbolt.org/z/WT3g9q