Protocol relative URL for MathJax

But when the protocol is `file:`, use the http link for the MathJax URL.

Sometimes web browsers do not allow https resources in http pages, and we may see errors like this when we hard-code https for the MathJax URL:

    Failed to load resource: Error creating SSL context ()

This is reproducible in the RStudio viewer on Windows and Linux. An example is the vignette `knitr-intro` after 

```r
devtools::install_github('yihui/knitr', build_vignettes = TRUE)
help(package = 'knitr')
```

The math expression is not rendered in the RStudio viewer.

@jjallaire If you think this change makes sense, I will make the same change in other templates. Or if you can "fix" the RStudio viewer, that will be perfect, too.