pmd: fix ignored property minimumLength of rule AvoidDuplicateLiterals

The rule AvoidDuplicateLiterals ignores its property minimumLength
because it is set not until after the source code has been analyzed.
This change sets the property before the analysis.
pmd: fix #1049 Errors in "How to write a rule"
Merge branch 'master' of github.com:pmd/pmd