ruby: remove minitest deps on gems

minitest can live without gems. Just a minor fix to
solve a require that fails when gem is missing
ruby: link ext/digest/* to openssl only when needed, fix rmd160 name

ext/digest/rmd160 was referencing a function that never existed in openssl.
The name was simply mistyped. Now it can use openssl.

openssl was always linked to ext/digest when library is avaiable,
even when it was disable by configure option and not used by code.

upstream refs: https://bugs.ruby-lang.org/issues/10252
upstream refs: https://bugs.ruby-lang.org/issues/10324