Fixes #13983

Two changes have been made:
- In AkkaBuild, forced using UTF-8 to verify HTML files containing
  diagrams
- In the documentation chapter Building Akka, added a section clarifying
  the Graphviz dependency of generating Scaladoc Content Hierarchies

These fixes eliminates the issues I faced while building Akka on Windows
with NetBeans. The first issue is a Windows specific issue where the
default charset cannot be UTF-8. The second is Windows related in the
sense that Graphviz, the optional dependency of Scaladoc, had to be
manually resolved.
Updates in the Building Akka documentation page in
accordance with the discussion with @bantonsson
=pro Update test dependencies to Scala 2.11 cross published versions
=pro #15045 Adjust build for Scala 2.11.0

* Forward port of 2b891e7
Merge pull request #15047 from akka/wip-update-test-dependencies-master-ban

=pro Update test dependencies to Scala 2.11 cross published versions
Merge pull request #15011 from csomogyi/master

Fixing minor issues I found while building Akka on Windows
=pro Remove unused dependency version
Merge pull request #15051 from akka/wip-remove-unused-dependency-version-ban

=pro Remove unused dependency version
Merge pull request #15049 from akka/wip-2.11-build-adjustments-master-patriknw

=pro #15045 Adjust build for Scala 2.11.0 (for validation)
=pro #15031 use sbt-unidoc plugin