Explicitly install Ruby before deploying

Otherwise the build fails with:

    $ rvm 2.7 --fuzzy do ruby -S gem install dpl -v 1.10.16
    Ruby ruby-2.7.2 is not installed.

.travis.yml

@@ -20,6 +20,9 @@ before_cache:
 cache:
     directories:
         - _build/dev
+
+before_deploy:
+  - rvm install 2.7
 deploy:
     provider: script
     skip_cleanup: true