Tweak travis and appveyor for Symfony 4

Author: nicolas-grekas

composer.json

@@ -20,14 +20,14 @@
         "symfony/polyfill-mbstring": "~1.0"
     },
     "require-dev": {
-        "symfony/config": "~2.8|~3.0|~4.0",
-        "symfony/intl": "^2.8.18|^3.2.5|~4.0",
-        "symfony/yaml": "~3.3|~4.0",
+        "symfony/config": "~3.4|~4.0",
+        "symfony/intl": "~3.4|~4.0",
+        "symfony/yaml": "~3.4|~4.0",
         "psr/log": "~1.0"
     },
     "conflict": {
-        "symfony/config": "<2.8",
-        "symfony/yaml": "<3.3"
+        "symfony/config": "<3.4",
+        "symfony/yaml": "<3.4"
     },
     "suggest": {
         "symfony/config": "",