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