Use a more recent version of gyoku

Since savonrb/gyoku#46 is very critical for us, and we depend on Savon, it would be nice to be able to get a Savon version that depends on a newer version of `gyoku`. Because the gem version is pinned in the gemspec, it's currently impossible to use the "fixed" version of Gyoku w/ Savon - hence, this PR.

Thanks in advance.