Proper fields alignment in InternalUuidGenerator struct

[vigdail]

Fixes #14
InternalUuidGenerator struct did not satisfy alignment requirements.

According to atomic package docs:

On ARM, 386, and 32-bit MIPS, it is the caller's responsibility to arrange for 64-bit alignment of 64-bit words accessed atomically via the primitive atomic functions (types Int64 and Uint64 are automatically aligned). The first word in an allocated struct, array, or slice; in a global variable; or in a local variable (because the subject of all atomic operations will escape to the heap) can be relied upon to be 64-bit aligned.

I swapped fields order, now the struct has proper alignment.

Tested on Raspberry Pi 2 Model B

[mayswind]

lgtm

[mayswind]

thanks