Instruct Ruby to fail the build if openssl or psych are missing

Normally, Ruby `make` step will print a warning about any missing
extensions, but will not abort the build and instead proceed as normal.

Since Ruby installations without openssl or psych are essentially
broken, ruby-build used to have a `verify_openssl` build step to test if
the newly built Ruby can load these extensions, and print helpful
information and abort the build on errors:

    Loading the Ruby openssl extension failed
    ERROR: Ruby install aborted due to missing extensions

The `verify_opensl` implementation was necessary to provide a good
experience for ruby-build users, but was hacky and I would prefer to
eliminate it.

It appears that passing `--with-ext=openssl,psych` to the Ruby configure
step marks those extensions as mandatory and fails the `make` process if
they failed to build. This is exactly the behavior we want, so this
enables the configure option for all Ruby builds.