Change in `options:` key parameters does not clear cache for rebuilds

[AakashGfude]

When you add/edit/remove a key from the options object in _toc.yml, the cached objects are still used on subsequent rebuilds.

The cache should be cleared and a fresh build should happen.

[AakashGfude]

Update:

The build is refreshed when you add numbered: True. Ex:

- caption: Tutorials
  numbered: True
  chapters:
  - file: start/your-first-book
    sections:
    - file: start/overview
    - file: start/create
    - file: start/build
    - file: start/publish
  - file: tutorials/references
  - file: basics/questions

but when you change it to numbered: False it does not refresh Ex:

- caption: Tutorials
  numbered: False
  chapters:
  - file: start/your-first-book
    sections:
    - file: start/overview
    - file: start/create
    - file: start/build
    - file: start/publish
  - file: tutorials/references
  - file: basics/questions

still gives:

[chrisjsewell]

hopefully closed in #51, but see my comment on #48

[chrisjsewell]

As mentioned, I think this is now fixed, but feel free to re-open if not