use ruby 2.7 for testing rails 5.2. (#950)
use ruby 2.7 for testing rails 5.2. (#950)

This is to see if it fixes an issue installing from github on travis
(maybe fixed in 2.6.7 - travis is on 2.6.6, but 2.7 will probably work
anyway)
thredded/thredded,thredded/thredded,thredded/thredded,thredded/thredded