Fixed the trailing L problem of hex(...)

[sxjscience]

Using hex for 16-bit string conversion will cause the Trailing L Problem for 64-bit value. See #545 for more detail.

[piiswrong]

Can someone verify that this works for both julia & python before we merge?

[vchuravy]

This only changes code on the python side of things, so no need for checking Julia

I can confirm that the python side works, by running the numpy_softmax example.

[pluskid]

This will not affect Julia side as the native operator for Julia is still pending for some refactoring of the async callback in libmxnet. In the current interface it is not possible to implement. On my phone but can provide reference to existing issues if needed.

[vchuravy]

See dmlc/MXNet.jl#16 for reference.