Move to 7.1 test since this will be 7.1 now

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>

.travis.yml

@@ -106,7 +106,7 @@ matrix:
       env: "DB=mysql SAUCE=TRUE SELENIUM_BROWSER=firefox"
     - php: 7.2
       env: "DB=mysql SAUCE=TRUE SELENIUM_BROWSER=chrome"
-    - php: 7.0
+    - php: 7.1
       env: "DB=mysql CORE_BRANCH=stable15"
     - php: 7.1
       env: "DB=mysql CORE_BRANCH=v15.0.0"