Remove ruby-head from travis config and add 2.7

Author: toduq

.travis.yml

@@ -5,7 +5,7 @@ rvm:
   - 2.4
   - 2.5
   - 2.6
-  - ruby-head
+  - 2.7
 before_install:
   - if [ ${TRAVIS_RUBY_VERSION} = '2.2' ]; then
       gem install bundler -v '< 2';