add a missing require "pathnme"

TTFunk::File.verify_and_open uses the Pathname keyword. Without this
require statement, one gets the following error

lib/ttfunk.rb:31:in `verify_and_open': uninitialized constant TTFunk::File::Pathname (NameError)

for example, when running the metrics.rb example.
Merge pull request #18 from prawnpdf/require-pathname

add a missing require "pathnme"