Use protocol-relative URL when :ssl is false

I don't think anybody hosting on HTTPS would ever want to not use HTTPS
because using HTTP will display a nasty warning to the user. Setting
:ssl to true still works the same as before, but now setting :ssl to
false(or not setting it at all as false is the default) will still use
HTTPS when the page is hosted over HTTPS.
Merge pull request #71 from nemurimasu/protocol-relative

Use protocol-relative URL when :ssl is false