Bump version number in build.number to 2.11.4

This should happen every time we cut a new release. We don't touch
versions.properties files because that file specifies dependencies and
we don't want to depend on broken Scala 2.11.3 release.
Merge pull request #4073 from gkossakowski/bump-version

Bump version number in build.number to 2.11.4
SI-8916 Fix -Ywarn-unused-import warnings
SI-8916 Further fixes with -Ywarn-unused included
SI-8916 Revert addition of -Ywarn-unused[-import]

  -Ywarn-unused-import: creates a bogues warning, see SI-7750

  -Ywarn-unused: creates a lot of noise & has bugs, see SI-7707, SI-7712