Update PDF Box (2.0)

[benjaminpick]

PDF Box finally allows to create thumbnails of a pre-defined size (2.0 is released now)

See https://issues.apache.org/jira/browse/PDFBOX-1270

Needs an update of the bundled PDFBox in regain as well.

[benjaminpick]

See Migration Guide:
https://pdfbox.apache.org/2.0/migration.html