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: facebook/hhvm#2944 (comment)

Closes #68

GitHub Author: Daniel Sloof <[email protected]>

@override-unit-failures

Test Plan: fbmake runtests 100%

Reviewed By: [email protected], [email protected]

FB internal diff: D1407426

folly/MacAddress.cpp

@@ -24,7 +24,7 @@ using std::string;
 
 namespace folly {
 
-const MacAddress MacAddress::BROADCAST{Endian::big(0xffffffffffffU)};
+const MacAddress MacAddress::BROADCAST{Endian::big(uint64_t(0xffffffffffffU))};
 const MacAddress MacAddress::ZERO;
 
 MacAddress::MacAddress(StringPiece str) {