Bump version to 2.15.0

doc/haddock.xml

haddock.cabal

haddock.spec

html-test/ref/A.html

html-test/ref/AdvanceTypes.html

html-test/ref/B.html

html-test/ref/Bold.html

html-test/ref/Bug1.html

html-test/ref/Bug195.html

html-test/ref/Bug2.html

html-test/ref/Bug3.html

html-test/ref/Bug4.html

html-test/ref/Bug6.html

html-test/ref/Bug7.html

html-test/ref/Bug8.html

html-test/ref/BugDeprecated.html

html-test/ref/BugExportHeadings.html

html-test/ref/Bugs.html

html-test/ref/CrossPackageDocs.html

html-test/ref/DeprecatedClass.html