build: set minimum php version to 7.3

Author: ecastro98

azure-pipelines.yml

@@ -3,7 +3,7 @@ pool:
   vmImage: 'ubuntu-latest'
 
 variables:
-  phpVersion: 7.1
+  phpVersion: 7.3
 
 steps:
   - script: |
@@ -17,4 +17,4 @@ steps:
   - script: composer install --no-interaction --prefer-dist
     displayName: 'composer install'
   - script: composer test
-    displayName: 'Run tests'
+    displayName: 'Run tests'

composer.json

@@ -6,7 +6,7 @@
     "minimum-stability": "dev",
     "prefer-stable": true,
     "require": {
-        "php": "^7.1",
+        "php": "^7.3",
         "ext-json": "^1.5",
         "php-http/client-implementation": "^1.0",
         "php-http/httplug": "^2.0",