Delete zipfile-bug.txt

The observed bug is probably solved (or should have a ticket), this 4-year-old thread dump does not seem to belong here.
Merge pull request #4073 from gkossakowski/bump-version

Bump version number in build.number to 2.11.4
Merge pull request #4071 from Blaisorblade/patch-3

Delete zipfile-bug.txt
Merge pull request #4077 from gkossakowski/bump-version

Bump version to 2.11.5 in build.number file.
Correct collections variable definitions to avoid many scaladoc warnings.