Update .travis.yml

Author: Tucker-Eric

.travis.yml

@@ -5,6 +5,7 @@ php:
   - 7.0
   - 7.1
   - 7.2
+  - 7.3
 
 env:
   global:
@@ -13,9 +14,10 @@ env:
 sudo: false
 
 before_install:
+  - echo "memory_limit=2G" >> ~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/travis.ini
   - travis_retry composer self-update
 
 install:
-  - composer install --no-interaction --prefer-dist
+  - COMPOSER_MEMORY_LIMIT=-1 travis_retry composer install --no-interaction --prefer-dist
 
 script: vendor/bin/phpunit