The AlignTab docs or the linked examples have enough info to cover what you need.
If you highlight the original text, then Cmd + Shift + P (or Ctrl Shift P on windows) and enter AlignTab Live Preview mode, you can enter a regex and use the special rlc syntax.
The syntax is:
\s+:/r0c1l
The regex are \s+ (greater than 1 spaces) to the first :, then r0 (align right, 0 spaces), c1 (center mark with 1 padding to the right), then l (left align the remaining text). There's no need for an 'f1' at the end of the expression since the regex matches the leading spaces before your first colon (per your example).
If you had newly written text without the pre-formatting that you've already done, like:
Version: 1.4.1
Author: Ken Wheeler
Website: http://kenwheeler.github.io
Docs: http://kenwheeler.github.io/slick
Repo: http://github.com/kenwheeler/slick
Issues: http://github.com/kenwheeler/slick/issues
Then to convert to your desired format below, will be:
:/r0c1lf1
Converted below:
Version: 1.4.1
Author: Ken Wheeler
Website: http://kenwheeler.github.io
Docs: http://kenwheeler.github.io/slick
Repo: http://github.com/kenwheeler/slick
Issues: http://github.com/kenwheeler/slick/issues
This result is actually better than your original question text, because there's no leading space.
P.S. I found that the Tablular examples from the AlignTab docs to be slighty more helpful, although it doesn't represent the real implementation of AlignTab.