This is a more educated assumption based on my experience with CSS and Javascript, but you can try something like this:
selector = 'input#billing\:street1' find(selector)
Note the backslash before the colon; this eludes the character in CSS. However, for Javascript this is a little different. You need two slashes to escape the character. For instance:
selector = 'input#billing\\:street1' find(selector)
I'm not sure which one could do the trick (if it was), since I have zero experience with Cucumber, Capybara and Poltergeist / PhantomJS, but based on your code, it looks as if you wanted to try a double first slash \\ .
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