print/ruby-pdf-core: update to 0.8.1

Update ruby-pdf-core pacakge to 0.8.1.

0.8.1 - 2018-04-28

Fixed

* Make sure stamp streams are fritable Alexander Mankuta
* Handle text rendering from forzen strings Alexander Mankuta

0.8.0 - 2018-04-27

Changed

* Minimum Ruby version is 2.3 Alexander Mankuta
* Trailing fraction zeroes are removed from numbers Alexander Mankuta

(taca)