Upgrade phpunit to ^7 (GoogleCloudPlatform#1000)

Author: bshaffer

appengine/flexible/analytics/composer.json

@@ -1,6 +1,6 @@
 {
     "require": {
-        "silex/silex": "^1.3",
+        "silex/silex": "^2.3",
         "twig/twig": "^1.24",
         "guzzlehttp/guzzle": "^6.3",
         "symfony/css-selector": "^3.1"

appengine/flexible/cloudsql-mysql/composer.json

@@ -1,6 +1,6 @@
 {
     "require": {
-        "silex/silex": "^1.3",
+        "silex/silex": "^2.3",
         "google/apiclient": "^2.0"
     },
     "require-dev": {

appengine/flexible/cloudsql-postgres/composer.json

@@ -1,6 +1,6 @@
 {
     "require": {
-        "silex/silex": "^1.3",
+        "silex/silex": "^2.3",
         "google/apiclient": "^2.0"
     },
     "require-dev": {

appengine/flexible/datastore/composer.json

@@ -1,6 +1,6 @@
 {
     "require": {
-        "silex/silex": "^1.3",
+        "silex/silex": "^2.3",
         "google/cloud-datastore": "^1.0"
     },
     "require-dev": {

appengine/flexible/helloworld/composer.json

@@ -1,10 +1,10 @@
 {
   "require": {
     "php": "5.6.*|7.0.*|7.1.*|7.2.*",
-    "silex/silex": "^1.3"
+    "silex/silex": "^2.3"
   },
   "require-dev": {
-    "symfony/browser-kit": "^3.0",
-    "symfony/http-kernel": "^3.0"
+    "symfony/browser-kit": "^4.4",
+    "symfony/http-kernel": "^4.4"
   }
 }

appengine/flexible/mailgun/composer.json

@@ -1,6 +1,6 @@
 {
     "require": {
-        "silex/silex": "^1.3",
+        "silex/silex": "^2.3",
         "mailgun/mailgun-php": "~2.0",
         "php-http/guzzle6-adapter": "^1.0",
         "symfony/yaml": "^3.1"

appengine/flexible/mailjet/composer.json

@@ -1,6 +1,6 @@
 {
     "require": {
-        "silex/silex": "^1.3",
+        "silex/silex": "^2.3",
         "mailjet/mailjet-apiv3-php": "^1.1",
         "guzzlehttp/guzzle": "~6.1.0"
     },

appengine/flexible/memcache/composer.json

@@ -1,6 +1,6 @@
 {
     "require": {
-        "silex/silex": "^1.3",
+        "silex/silex": "^2.3",
         "twig/twig": "^1.24"
     },
     "require-dev": {

appengine/flexible/metadata/composer.json

@@ -1,6 +1,6 @@
 {
     "require": {
-        "silex/silex": "^1.3",
+        "silex/silex": "^2.3",
         "google/cloud-core": "^1.5"
     }
 }

appengine/flexible/sendgrid/composer.json

@@ -1,6 +1,6 @@
 {
     "require": {
-        "silex/silex": "^1.3",
+        "silex/silex": "^2.3",
         "sendgrid/sendgrid": "^5.0"
     },
     "require-dev": {