freshen up the README

[flavorjones]

The README needs some love. I'd like to:

• convert it from RDoc to Markdown
• remove the travis badge
• update the dependencies section to match update gem dependency versions #565 and drop support for EOLed Rubies (< 2.5) #563
• docs.seattlerb.org is not up-to-date. let's start pointing people somewhere they'll be updated.

[mmulet]

I made a code relay task for this https://github.com/code-relay-io/mechanize/blob/main/README.rdoc . We will get it done.

[flavorjones]

@mmulet Thank you! I'm really interested to see code relay in action.

[flavorjones]

@mmulet Someone submitted a PR for this already; but thanks for trying to invoke Code Relay on this.