drop ruby 1.9.x support for Chef-12

Ruby 1.9.3 is in security-only maintenance phase which means that
bugs for the core ruby interpreter will not be fixed and Chef cannot
support ruby 1.9.x.  Users should be using omnibus-chef packages
which come with a built-in ruby 2.x or should upgrade their own
installations to ruby 2.x (and accept that ongoing upgrade and
support cost).  Chef 11 will continue to be shipped on ruby 1.9.x
until Chef 11 is EOL.
update docs for dropping support of 1.9.3
Remove 1.9 from travis, bump 2.1 to .5 patch release
Force TK to install prerelease Chef pkg for now

In the future (after Chef 12.0.0 is released) we should remove this. For
now, we need to force TK to install a prerelease version of the Chef
package, so it will include Ruby 2.1, and therefore not choke on
our requirement for Ruby >= 2.0