Change php version for travis to check against. (#128)

* Change php version for travis to check against.
Fix issues #126 produced by the inclusion of phpstan/extension-installer
* Add phpunit-polyfills to dev packages

Author: lauramelos

.travis.yml

@@ -4,9 +4,9 @@ sudo: false
 
 # Test main supported versions of PHP and HHVM against latest WP. 5.2 is min supported version.
 php:
-  - 7.0
   - 7.1
   - 7.2
+  - 7.3
 
 services:
   - mysql

composer.json

@@ -24,7 +24,8 @@
         "woocommerce/woocommerce-sniffs": "0.1.0",
         "php-stubs/woocommerce-stubs": "^5.1",
         "szepeviktor/phpstan-wordpress": "^0.7.5",
-        "phpstan/extension-installer": "^1.1"
+        "phpstan/extension-installer": "^1.1",
+        "yoast/phpunit-polyfills": "^1.0"
     },
     "require": {
         "amzn/amazon-pay-api-sdk-php": "^2.2"