These days with Github and Bundler you don't have to be a maintainer in order to fix issues in the gems you're using. Just fork the project on Github, fix whatever issue is there, and them simply point Bundler at your fork. If it is a handy extension to the gem rather than just a hack for yourself you might also consider making a pull request to the original maintainer to roll your changes into the official gem.
Having said all of that, the issue that might cause this problem doesn't seem to be present in the gemspec of the 0.0.10 version of webrobots. This issue can occur when the date in the gemspec is in the format that you have in your error e.g.:
"2011-07-01 00:00:00.000000000Z"
Looking at a snippet of gemspec though we have:
s.authors = [%q{Akinori MUSHA}]
s.date = %q{2011-07-01}
s.description = %q{This library helps write robots.txt compliant web robots in Ruby.
So that seems to be correct. Another issue that has been known to cause this problem is a YAML parser issue, to fix you need to force the use of a particular YAML parser (psych). You need to define the following somewhere:
require 'yaml'
YAML::ENGINE.yamler = 'syck'
Lastly you might want to clean your local installation and then do a redeploy which may sort you out. Once again if you're using Bundler something like this:
gem update --system
rvm gemset empty mygemset
gem install bundler
gem install