Try something

.travis.yml

@@ -11,12 +11,7 @@ matrix:
   fast_finish: true
   include:
     - php: "5.6"
-      # proc_open(): fork failed - Cannot allocate memory
-      env: COMPOSER_FLAGS="--prefer-source"
-    - php: "7.0"
-    - php: "7.1"
-    - php: "7.2"
-      env: CHECK_CS=true
+
 
 cache:
   directories:
@@ -31,11 +26,12 @@ branches:
 # setup requirements for running unit tests
 before_script:
   # Disable xdebug and remove memory limit to speed things up
+  - composer self-update --rollback
   - phpenv config-rm xdebug.ini
   - echo "memory_limit=-1" >> ~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/travis.ini
   - php -v
   # Install packages using composer
-  - travis_retry composer install  --no-interaction --no-progress $COMPOSER_FLAGS
+  - composer install --no-interaction --no-progress $COMPOSER_FLAGS
 
 # execute phpunit as the script command
 script: