Fix build?

.travis.yml

@@ -1,3 +1,4 @@
+dist: bionic
 language: php
 sudo: false
 notifications:
@@ -9,13 +10,14 @@ notifications:
 branches:
   only:
   - dev
+  - hotfix
   - "/^(0|[1-9]\\d*)\\.(0|[1-9]\\d*)\\.(0|[1-9]\\d*)(-(0|[1-9]\\d*|\\d*[a-zA-Z-][0-9a-zA-Z-]*)(\\.(0|[1-9]\\d*|\\d*[a-zA-Z-][0-9a-zA-Z-]*))*)?(\\+[0-9a-zA-Z-]+(\\.[0-9a-zA-Z-]+)*)?$/"
 php:
-- 7.1
-- 7.2
-- nightly
+  - 7.1
+  - 7.2
+  - 7.3
 env:
-- TRAVIS_NODE_VERSION="8"
+- TRAVIS_NODE_VERSION="10"
 matrix:
   allow_failures:
   - php: nightly
@@ -23,7 +25,7 @@ cache:
   apt: true
   directories:
     - "$HOME/.composer/cache"
-  - "$HOME/.yarn-cache"
+    - node_modules
     - vendor
 before_install:
 - composer self-update

composer.json

@@ -24,12 +24,13 @@
 	},
 	"require": {
 		"php": ">=7.1",
-		"composer/installers": "~1.0",
+		"composer/installers": "~1.3.0",
 		"pressbooks/mix": "^2.1",
 		"soberwp/intervention": "1.2.0-p",
 		"spatie/color": "^1.1"
 	},
 	"require-dev": {
+		"phpunit/phpunit": "~6.5",
 		"pressbooks/coding-standards": "^1.0",
 		"wpreadme2markdown/wp2md": "^3.0"
 	},