Create a custom swap to avoid error

.travis.yml

@@ -48,24 +48,32 @@ branches:
     except:
         - legacy
 
+before_install:
+    - if [[ $TRAVIS_REPO_SLUG = wallabag/wallabag ]]; then cp .composer-auth.json ~/.composer/auth.json; fi;
+
+install:
+    - if [[ $ASSETS = build ]]; then source ~/.nvm/nvm.sh && nvm install 6.10; fi;
+    - if [[ $ASSETS = build ]]; then npm install -g yarn@latest; fi;
+    - if [[ $ASSETS = build ]]; then yarn install; fi;
+
 before_script:
     - PHP=$TRAVIS_PHP_VERSION
     - if [[ ! $PHP = hhvm* ]]; then echo "memory_limit=-1" >> ~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/travis.ini; fi;
     # xdebug isn't enable for PHP 7.1
     - if [[ ! $PHP = hhvm* ]]; then phpenv config-rm xdebug.ini || echo "xdebug not available"; fi
     - composer self-update --no-progress
     - if [[ $DB = pgsql ]]; then psql -c 'create database wallabag_test;' -U postgres; fi;
-
-install:
-    - if [[ $ASSETS = build ]]; then source ~/.nvm/nvm.sh && nvm install 6.10; fi;
-    - if [[ $ASSETS = build ]]; then npm install -g yarn@latest; fi;
-    - if [[ $ASSETS = build ]]; then yarn install; fi;
-
-before_install:
-    - if [[ $TRAVIS_REPO_SLUG = wallabag/wallabag ]]; then cp .composer-auth.json ~/.composer/auth.json; fi;
+    # increase swap to avoid "proc_open(): fork failed - Cannot allocate memory"
+    # this should be removed when no more PHP 5 build will be defined
+    - sudo swapon -s
+    - sudo fallocate -l 4G /swapfile
+    - sudo chmod 600 /swapfile
+    - sudo mkswap /swapfile
+    - sudo swapon /swapfile
+    - sudo swapon -s
 
 script:
-    - COMPOSER_MEMORY_LIMIT=-1 travis_wait bash composer install -o  --no-interaction --no-progress --prefer-dist
+    - travis_wait bash composer install -o --no-interaction --no-progress --prefer-dist
     - echo "travis_fold:start:prepare"
     - make prepare DB=$DB
     - echo "travis_fold:end:prepare"