Use Qt 5.7.1 PPA

Thanks for moving the target all the time, lads.

.travis.yml

@@ -49,7 +49,7 @@ cache:
 before_install:
 - >
     if [[ ${TRAVIS_OS_NAME} = linux ]]; then
-        sudo add-apt-repository --yes ppa:beineri/opt-qt57-trusty
+        sudo add-apt-repository --yes ppa:beineri/opt-qt571-trusty
         sudo apt-get update -qq
         sudo apt-get install -qq qt57base qt57gamepad
         if [[ "$COMPILER" = "g++-5" ]]; then