Try to stay compatible with ruby-head

.travis.yml

@@ -5,17 +5,12 @@ rvm:
   - 2.4
   - 2.5
   - &latest_ruby 2.6
-  - 2.7
-  - ruby-head
 
 matrix:
   include:
     - rvm: *latest_ruby
       script: bundle exec rake memory_profile:run
       name: Profiling Memory Usage
-  allow_failures:
-    - rvm: ruby-head
-    - rvm: 2.7
 
 branches:
   only: