Update pre-release versions for current recommendations

According to Semver 2.0.0 section 11, the precedence for pre-releases
should take the form of `-rc.1` instead of `-rc1`

The reason for this is that version precedence without the dots would be
1.0.0-rc10 < 1.0.0-rc2 but with the dots would be 1.0.0-rc.2 <
1.0.0-rc.10 which is intuatively more correct.
Merge pull request #1 from hunner/update_prerel

Update pre-release versions for current recommendations