Update downloads and libraries page (en)

**Not for merging**

Some information was outdated:

- mention of specific Ruby versions
- new default behaviour of the "gem" command (--remote)

"MRI" had been mentioned, but not explained.

I would re-introduce the hint regarding the Ruby license. It had been removed some time ago. Why?

I don't know whether the section on Mac OS X is still valid.

I'm sure the section on Solaris is oudated, but I can't fix it. (And the first sentence doesn't make any sense to me.)

@postmodern, @chikamichi, @JEG2 Feedback?