Remove ruby 1.9.2 from travis build matrix

[unasuke]

• RVM not support 1.9.2 with --binary flag
  • https://travis-ci.org/matthewrudy/memoist/jobs/606960866
• Minor version is duplicated with 1.9.3

$ docker run -it --rm buildpack-deps:buster bash
root@66cac41f18c4:/# curl -sSL https://get.rvm.io | bash -s stable
# ...snip...
root@66cac41f18c4:/# source /etc/profile.d/rvm.sh
root@66cac41f18c4:/# rvm use 1.9.2 --install --binary --fuzzy
Required ruby-1.9.2 is not installed - installing.
Searching for binary rubies, this might take some time.
Requested binary installation but no rubies are available to download, consider skipping --binary flag.
Gemset '' does not exist, 'rvm ruby-1.9.2 do rvm gemset create ' first, or append '--create'.

[unasuke]

Why fail CI on jruby-9.1.5.0 ...? :thinking: :question:

[sebjacobs]

Given that rvm no longer provides a 1.9.2 binary and that we're already testing against 1.9.3 this seems like the right call to me.