Gemfile: use https://rubygems.org

The older `:rubygems` source has been deprecated.
Update css_parser to 1.3.5
Deadweight#strip strips whitespace

Selectors like `.foo ::-webkit-input-placeholder` were causing
Nokogiri to throw an exception. Specifically,
`Deadweight#strip` would remove the `::-webkit-input-placeholder`,
leaving a trailing whitespace, which Nokogiri doesn't like.
Merge branch 'james-strip'