Fix alignment of 64bit platforms

[isanych]

gcc 5 sanitizer was complaining about alignment on 64bit platform:

pbjson/rapidjson/document.h:1248:9: runtime error: member call on misaligned address 0x631000028b6c for type 'struct GenericValue', which requires 8 byte alignment
0x631000028b6c: note: pointer points here
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[coveralls]

Coverage remained the same at 99.82% when pulling 1576cde on isanych:master into ba76475 on miloyip:master.

[miloyip]

Note that, this will increase the sizeof(Value) from 20 bytes to 24 bytes on 64-bit platform.
But I think this would be safer in unknown/future 64-bit architecture.
I will also try to optimize it to 16 bytes on some platforms later, where only 48 bits address space is actually used.