only rewrite current_version key of bumpconfig file

[briancappello]

Hi,

I noticed that whenever I wanted to bump a version, the script would actually completely rewrite my setup.cfg file, which was kind of very unexpected. This PR makes sure that doesn't happen. However I'm a new user of bumpversion, so I don't know if this change might break other option combinations (for example, after this PR, current_version is a required key in the bump config.. maybe it was before, maybe not)