Be explicit about what we're passing to Endian::big

Be explicit about what we're passing to Endian::big

Summary:
This prevents a linker error on OSX:
```
Undefined symbols for architecture x86_64:
"folly::detail::EndianIntBase::swap(unsigned long)",
referenced from:
__GLOBAL__sub_I_MacAddress.cpp in libfolly.a(MacAddress.cpp.o)
ld: symbol(s) not found for architecture x86_64
```

We need folly and third-party changes in order to land a pull request for HHVM
that starts to get FastCGI running on OSX.

See the checklist of the HHVM pull request here: https://github.com/facebook/hhvm/pull/2944#issuecomment-47281003

Closes #68

GitHub Author: Daniel Sloof <goapsychadelic@gmail.com>

@override-unit-failures

Test Plan: fbmake runtests 100%

Reviewed By: pt@fb.com, njormrod@fb.com

FB internal diff: D1407426