63083948 pdf source code

The source-code font size was wrong in PDFs, it took me more than an hour to figure out why. Turns out the problem was moving the style file 'softcover.sty' to a subdirectory; there was one place in the `polytexnic` gem where this location was hard-coded, and moving the file silently broke things. It seems like it serves me right for hard-coding it, but it's trickier than it seems because the config lives in `softcover`, and the only way I know of to get  it is to include `softcover` in `polytexnic`, but this is a circular dependency. In any case, I've hard-coded the correct file path and, more importantly, [added a test to the `softcover` gem](https://github.com/softcover/softcover/pull/83) to catch this problem if it ever recurs.