don't wrap shell args in quotes now that we're using shellescape

From the [shellescape documentation](http://www.ruby-doc.org/stdlib-1.9.3/libdoc/shellwords/rdoc/Shellwords.html#method-c-shellescape):

> Note that a resulted string should be used unquoted and is not intended for use in double quotes nor in single quotes.

This is not a merely academic detail, as the quoting used prior to this change means that paths with spaces will fail when outputting PDFs.