Fix (spurious) test failure on big-endian architectures in Ruby specs

Our example includes a int_64_t value (the PDU's length prefix), which
is endian-sensitive and so needs to be asserted using native endianness.

Originally reported as a bug on the upstream project where I initially
prototyped the Watchman protocol Ruby implementation. References:

- https://github.com/wincent/Command-T/issues/101
- https://bugzilla.redhat.com/show_bug.cgi?id=1130079