Added fixes discussed in PR #70

- Updated all font files from master of Fira [repo](https://github.com/mozilla/Fira), now using Fira 3.111.  **Note: There is no longer any FiraSans-RegularItalic.**  We don't use that in the PDF generation, but I believe Andy and Ishac set it up to be used in the web book.
- Removed margin settings, allowing defaults for `srcbook` to take over laying out the pages.
- Added `breakatwhitespace=true` and `literate={\_}{}{0\discretionary{\_}{}{\_}}` to the listings environment setting to handle breaking code blocks at whitespace and at underscores (re #70).