pin ffi_yajl to 1.0.0 with semver constraint

0.1.5 is broken when JSON.pretty_generate(foo, nil) is called with
an explicit nil, so we have an absolute lower floor there on working
correctly.

We should release a 1.0.0 of ffi-yajl and pin to '~> 1.0' before
relesing the 11.14.0 final release.