Remove Ruby-specific parts of docs in prep for move to Go

- Ruby is not required at runtime anymore
- Most installation instructions are now obsolete. Will need to write
  new instructions when we distribute precompiled binaries.
- Remove tedious parts about slowness. It's not slow anymore!
Stop saying that non-git-related features will be rejected

As the Go version of hub matures, we plan to add features that are not
necessarily tied to git operations, such as viewing and managing issues.

Previously this wan't in the scope of the project. Now the scope widens.
Merge pull request #605 from github/rubyless-docs

Remove Ruby-specific parts of docs in prep for move to Go