Add an OSX workaround for sed in bump-version

[brrygrdn]

OSX doesn't tolerate -i without passing a blank argument. I think this is ok on 'nix but I could use a double-check.

[mwaddell]

Passing the blank argument blows up on Linux. You can either do:

sed -i.bak "s|dependabot/fetch-metadata@v[0-9.]*|dependabot/fetch-metadata@v${new_version}|g" "README.md"
rm README.md.bak

or if sed with in-place editing is not well supported across platforms, we could use PERL:

perl -pi -e "s/dependabot\/fetch-metadata\@v[0-9.]*/dependabot\/fetch-metadata\@v${new_version}/g" "README.md"

[brrygrdn]

Ah, cheers I hadn't had a chance to check on a VM. I'll update 👍🏻

[brrygrdn]

This is now fixed in #205