Keep only .afm files regarding fonts

Prawn does not use the *.ttf fonts internally so it's better not to include them in the final package. Prawn uses and offers developers build-in fonts using *.afm files, but there is no build-in support for .ttf fonts and in general it's a bad idea to include them.

Should solve font files licensing issue [1].

[1] https://github.com/prawnpdf/prawn/issues/474