add Ruby 1.9.2 to .travis.yml

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extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/trunk%4013354
ensure issue pdf test has relations (#17570)

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extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/trunk%4013357
use rbpdf gem instead of bundled rfpdf (#17570)

Contributed by Jun NAITOH.

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extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/trunk%4013358
remove general_pdf_encoding from lib/redmine/export/pdf.rb (#17570)

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extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/trunk%4013359
remove general_pdf_encoding from locale yamls (#17570)

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extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/trunk%4013360