Speed up nokogiri installation during Travis runs

Version 1.6 of Nokogiri builds its own copy of libxml2 during installation.
Setting `NOKOGIRI_USE_SYSTEM_LIBRARIES=true` skips this step and shaves about a
minute off of Travis runs.
Merge pull request #3746 from Sharpie/travis-speed-bump

Speed up nokogiri installation during Travis runs
synced_folders\rsync: remove non-portable '-v' flag from chown [GH-3743]

Aside of the already fixed OpenBSD, at least Solaris `chown` seems not
to support the `-v` option, so remove it from all guests.
Fix slash in CHANGELOG