Add Ruby 2.7 to Travis build matrix

Also, use latest rubies

Author: pirj

.travis.yml

@@ -48,13 +48,15 @@ matrix:
     rvm: jruby-head
 
   include:
-    # Rails 6 builds
+    # Rails 6.0 builds >= 2.5.0
     - rvm: jruby-head
       jdk: oraclejdk11
       env:
         - RAILS_VERSION='~> 6.0.0'
         - JRUBY_OPT=--dev
         - JAVA_OPTS="--add-opens java.base/sun.nio.ch=org.jruby.dist --add-opens java.base/java.io=org.jruby.dist --add-opens=java.base/java.lang=ALL-UNNAMED --add-opens=java.base/java.security=ALL-UNNAMED --add-opens=java.base/java.util=ALL-UNNAMED --add-opens=java.base/java.security.cert=ALL-UNNAMED --add-opens=java.base/java.util.zip=ALL-UNNAMED --add-opens=java.base/java.lang.reflect=ALL-UNNAMED --add-opens=java.base/java.util.regex=ALL-UNNAMED --add-opens=java.base/java.net=ALL-UNNAMED --add-opens=java.base/java.io=ALL-UNNAMED  --add-opens=java.base/java.lang=ALL-UNNAMED --add-opens=java.base/javax.crypto=ALL-UNNAMED --add-opens=java.management/sun.management=ALL-UNNAMED"
+    - rvm: 2.7.1
+      env: RAILS_VERSION='~> 6.0.0'
     - rvm: 2.6.6
       env: RAILS_VERSION='~> 6.0.0'
     - rvm: 2.5.8