Another try to fix bundler problems on travis

.travis.yml

@@ -1,14 +1,10 @@
 language: ruby
 
 rvm:
+  - 2.6.0
+  - 2.5.3
   - 2.4.5
   - 2.3.8
-  - 2.2.10
-
-matrix:
-  include:
-    - rvm: '2.2.10'
-      env: INSTALL_BUNDLER='1'
 
 env:
   - REDMINE_VER=4.0-stable DB=postgresql
@@ -30,7 +26,6 @@ before_install:
   - ln -s $TRAVIS_BUILD_DIR $REDMINE_PATH/plugins/$PLUGIN_NAME
   - cp $TRAVIS_BUILD_DIR/test/support/additional_environment.rb $REDMINE_PATH/config/
   - cp $TRAVIS_BUILD_DIR/test/support/database-$DB-travis.yml $REDMINE_PATH/config/database.yml
-  - if [[ "$INSTALL_BUNDLER" == '1' ]]; then gem install bundler; fi
 
 before_script:
   - psql -c 'create database travis_ci_test;' -U postgres