Font-size fix

We're using the REM method in CastleCSS.

HTML has 62.5% font-size so you can easily use REM like this:

p {
// 10 relative pixels
font-size: 1rem
}

code {
// 15 relative pixels
font-size: 1.5rem
}

There was a bug in castlecss-core, fixed this in 1.3.1