set_backend(AF_BACKEND_CUDA): Failed to load dynamic library on Windows

I got the lib folder in the system path, however:
```
julia> set_backend(AF_BACKEND_CUDA)
ERROR: ArrayFire Error (501) : Failed to load dynamic library.
Error in af_err __cdecl unified::AFSymbolManager::setBackend(af_backend)
In file src\api\unified\symbol_manager.cpp:232
Failed to load dynamic library. See http://www.arrayfire.com/docs/unifiedbackend.htm for instructions to set up environment for Unified backend.

Stacktrace:
[1] _error(::UInt32) at C:\Users\user\.julia\v0.6\ArrayFIre\src\util.jl:81
[2] set_backend(::UInt32) at C:\Users\user\.julia\v0.6\ArrayFIre\src\wrap.jl:865
```
```
julia> versioninfo()
Julia Version 0.6.3
Commit d55cadc350* (2018-05-28 20:20 UTC)
Platform Info:
OS: Windows (x86_64-w64-mingw32)
CPU: Intel(R) Core(TM) i7-3630QM CPU @ 2.40GHz
WORD_SIZE: 64
BLAS: libopenblas (USE64BITINT DYNAMIC_ARCH NO_AFFINITY Sandybridge)
LAPACK: libopenblas64_
LIBM: libopenlibm
LLVM: libLLVM-3.9.1 (ORCJIT, ivybridge)
```

```
julia> afinfo()
ArrayFire v3.6.1 (OpenCL, 64-bit Windows, build b443e14)
[0] NVIDIA: GeForce GT 630M, 2048 MB
-1- INTEL: Intel(R) HD Graphics 4000, 1400 MB
-2- INTEL: Intel(R) Core(TM) i7-3630QM CPU @ 2.40GHz, 6037 MB
```