Tweak travis and appveyor for Symfony 4

Author: nicolas-grekas

composer.json

@@ -17,14 +17,14 @@
     ],
     "require": {
         "php": "^7.1.3",
-        "symfony/filesystem": "~2.8|~3.0|~4.0"
+        "symfony/filesystem": "~3.4|~4.0"
     },
     "require-dev": {
-        "symfony/yaml": "~3.0|~4.0",
-        "symfony/dependency-injection": "~3.3|~4.0"
+        "symfony/yaml": "~3.4|~4.0",
+        "symfony/dependency-injection": "~3.4|~4.0"
     },
     "conflict": {
-        "symfony/dependency-injection": "<3.3"
+        "symfony/dependency-injection": "<3.4"
     },
     "suggest": {
         "symfony/yaml": "To use the yaml reference dumper"