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Set User-Agent by gulp, not using GM_info or GM_getMetadata

Some userscript engines don't support either GM_info or GM_getMetadata.
Ref: https://github.com/scriptish/scriptish/wiki/Comparison-Table
Set `private: true` in package.json
Update railscasts.user.css for redesign of rubygems.org

And rename railscast.user.css to railscasts.user.css (adding "s").

I can't find out what syntax highlight engine GitHub uses.
0.3.1