remove AFM monkey patch

* the same data is available via a accessor
add some failing specs for extracting text from symbol fonts
explictly require yaml for ruby2 happiness
Zapf Dingbat and Symbol fonts with no explicit encoding should default to the correct encodings

* fixes text extraction from these fonts
GlyphHash should recognise names with AXXXXX

* where X is a digit
Make explicit the assumption that bytes <= 255 remain unchanged

* The Encoding class has long assumed that unless otherwise configured,
  the bytes <= 255 remain unchanged when converting from glyph codes to
  unicode code points
* make that slightly more explicit be setting up the @mapping instance
  var with noop mappings
lookup AFM font metrics via glyph code instead of name

* lookups by glyph code work consistently across all fonts, including
  Symbol and Zapf Dingbats