ci: Fasten up CI testing

By now TravisCI downloads all packages while testing.
This costs some time especially for resolving versions.
Therefor we now cache the composer cache dir
and be more specific about dev-packages.

* .travis.yml:
  * Caching composer package cache
  * Swap WordPress download to composer to have it in cache
* composer.json:
  * Lowering tree of possible version to make resolver faster
  * Keep WordPress in cache too now

.travis.yml

@@ -46,13 +46,12 @@ cache:
 before_install:
   - mysql -e 'CREATE DATABASE dev;'
   # Try supporting other versions
-  - composer require --update-with-dependencies phpunit/phpunit:$PHPUNIT_VERSION
+  - composer require --update-with-dependencies phpunit/phpunit:$PHPUNIT_VERSION johnpbloch/wordpress:$WP_VERSION
 
 install:
   - composer install
   # Using WP-CLI instead of Composer because this way we can use the env matrix (above).
-  - vendor/bin/wp --allow-root core download --version=$WP_VERSION --force
-  - vendor/bin/wp --allow-root config create --force --dbuser=travis --dbpass="" --dbhost="127.0.0.1"
+  - vendor/bin/wp --allow-root config create --force --dbuser=travis --dbpass="" --dbhost="127.0.0.1" --skip-check
   - vendor/bin/wp --allow-root core install --skip-email
   # We get the latest patch version here to have less maintenance on the env matrix above.
   - vendor/bin/wp --allow-root core update --minor

composer.json

@@ -8,6 +8,7 @@
     }
   },
   "require": {
+    "johnpbloch/wordpress": "5.2.*",
     "pimple/pimple": "^3.0",
     "psr/container": "1.*"
   },
@@ -25,5 +26,8 @@
   },
   "config": {
     "sort-packages": true
+  },
+  "extra": {
+    "wordpress-install-dir": "srv"
   }
 }