changed pdfkit* related files in the gemspec to PDFKit* to avoid a warning on heroku
changed pdfkit* related files in the gemspec to PDFKit* to avoid a warning on heroku

pdfkit/pdfkit,asomanescu/pdfkit,christhekeele/pdfkit,toggl/pdfkit,centresource/pdfkit,gdi2290/pdfkit,christhekeele/pdfkit,globalxolutions/pdfkit,ContinuityControl/pdfkit,globalxolutions/pdfkit,cdwort/pdfkit,asomanescu/pdfkit,dimko/pdfkit