Turn on `--enable-shared` by default for all supported MRI Rubies

Some gems need the host ruby to have shared library enabled. Currently,
we're not aware of any potential downsides to having this enabled by
default.

> Shared libraries are libraries that are loaded by programs when they
> start. When a shared library is installed properly, all programs that
> start afterwards automatically use the new shared library.
> [...] if you have a program that needs a shared library (in this case ruby
> libruby.so.2.2.0 or similar) the program will fail if it doesn't exist.