bump: double-quote bumped version strings, avoid trailing newline

[imjasonh]

Before this change, melange bump foo.yaml 1.2.3 would produce:

package:
  name: foo
  version: 1.2.3

  epoch: 0

Note the blank line, which would trip up on our yam config.

With this change, melange bump foo.yaml 1.2.3 produces:

package:
  name: foo
  version: "1.2.3"
  epoch: 0

Which is functionally equivalent, without the trailing empty line.

I tried other styles, but none worked but this. I'm open to hearing better alternatives.