Eliminate the need to list version in gemspec.

Extract it from the run of jsduck --version.
Move JSDuck version number to separate file.

So it's easy to find and update in there.
Create Rake bump task to update version number.

rake bump

    Bumps the minor version number by one.

rake bump["6.0.0"]

    Bumps to the specified version.

The version.rb file gets updated and committed.  The commit gets tagged
with the same version number.
Fix Changelog link in README.
Refresh the list of contributors in README.

List all the contributors + those who have posted 5 or more issues.