WL-3356 Don’t leak memory when indexing PDFs.

PDFBox keeps a cache of something todo with fonts and over time this just accumulates heap. To work around this we clear out the cache after indexing each document. I’ve also bumped up the version of PDFBox to the latest version.
Merge pull request #7 from ox-it/WL-3356

WL-3356 Don’t leak memory when indexing PDFs.