Pdfplumber

Added
- Caching for `.decimalize()` method

Changed
- Upgrade to `pdfminer.six==20181108`
- Make whitespace checking more robust (PR [88](https://github.com/jsvine/pdfplumber/pull/88))

Fixed
- Fix issue [75](https://github.com/jsvine/pdfplumber/issues/75) (`.to_image()` custom arguments)
- Fix issue raised in PR [77](https://github.com/jsvine/pdfplumber/pull/77) (PDFObjRef resolution), and general class of problems
- Fix issue [90](https://github.com/jsvine/pdfplumber/issues/90), and general class of problems, by explicitly typecasting each kind of PDF Object

Fixed
- Fix bug in which, when calling get_page_image(...), the alpha channel could make the whole page black out.

Fixed
- Fix issue [67](https://github.com/jsvine/pdfplumber/issues/67), in which bool-type metadata were handled incorrectly

Fixed
- Fix issue [53](https://github.com/jsvine/pdfplumber/issues/53), in which non-decimalize-able (non_)stroking_color properties were raising errors.

Added
- `.travis.yml`, but failing on `.to_image()`

Changed
- Move from defunct `pycrypto` to `pycryptodome`
- Update `pdfminer.six` to `20170720`

Fixed
- Fix issue [41](https://github.com/jsvine/pdfplumber/issues/41), in which PDF-object-referenced cropboxes/mediaboxes weren't being fully resolved.