Avoid regex errors when running on Windows.

[sjp]

Hi,

A user of one of my packages emailed me with the following error message:

Error in grepl(paste("^", root, url, .Platform$file.sep, sep = ""), file_path) : 
  invalid regular expression '^C:\Users\USER\Documents\R\win-library\3.1\pkg\example/brew/', reason 'Invalid back reference'

This appeared to be caused by .Platform$file.sep being a backref in regex-land when used on Windows (i.e. .Platform$file.sep is \.

I made a minor change so that there is a char vector substring match instead of a regular expression (should be faster too, not that it matters).

Unfortunately I was unable to test whether this fixes the problem (don't have a Windows machine handy), but it does not break anything in Linux anyway.