fix version constraint for failing symfony 4.0

Author: ElectricMaxxx

.travis.yml

@@ -37,7 +37,7 @@ env:
 matrix:
   include:
     - php: 7.1
-      env: DEPS=dev SYMFONY_VERSION=^4@dev
+      env: DEPS=dev SYMFONY_VERSION=^4.0@dev
     - php: 7.1
       env: DEPS=dev SYMFONY_VERSION=^3.4@dev
     - php: 7.1
@@ -49,7 +49,7 @@ matrix:
   fast_finish: true
   allow_failures:
     - php: 7.1
-      env: DEPS=dev SYMFONY_VERSION=^4@dev
+      env: DEPS=dev SYMFONY_VERSION=^4.0@dev
     - env: TEST_INSTALLATION=true