Update or revert FFI gem

[headius]

In #6747 we updated JRuby 9.2 to have the same FFI guts as master, which then allows it to use the gem contents. This required a prerelease of FFI 1.15.4 to update the version check in that library; without this the gem's files would not load on 9.2.20.0.

When #6747 was merged to 9.2, and eventually master, this prerelease gem dependency went into lib/pom.rb. For the 9.3 release, we need to either update to the as-yet-unreleased (ffi/ffi#915) 1.15.4, or revert to 1.15.3.