add in / fix handling of fallback fonts

[urkle]

This PR adjusts the width calculation to utilize the new method added in PR prawnpdf/prawn#1259 so that fallback_fonts are considered when determining the width of a piece of text.

potential issues (to be resolved in a later PR in the core prawn)

• this uses a hard-coded 10,000 width when calculating the max width. If a PDF of greater than that width is ever created this would not correctly work.
  • in order to fix this, a better method would need to be created in the prawn core that would calculate the width with fallback fonts and NOT do line-wrapping. (would require some refactoring in the core).

TODO

• increase minimum required version of prawn once the other PR is released.