Exclude jruby-head with Rails main

Because jruby-head supports Ruby 2.6, not Ruby 2.7.

https://travis-ci.com/github/rsim/ruby-plsql/jobs/501530472

```
activesupport-7.0.0.alpha requires ruby version >= 2.7.0, which is incompatible
with the current version, ruby 2.6.5p0 (jruby 9.3.0.0-SNAPSHOT)
```

Author: yahonda

.travis.yml

@@ -52,6 +52,8 @@ gemfile:
 
 matrix:
     exclude:
+       - gemfile: gemfiles/Gemfile.activerecord-main
+         rvm: jruby-head
        - gemfile: gemfiles/Gemfile.activerecord-main
          rvm: jruby-9.2.17.0
        - gemfile: gemfiles/Gemfile.activerecord-main