Switch Linux builds to 5.7

.travis.yml

@@ -89,9 +89,9 @@ before_install:
 install:
   # linux dependencies: qt
   - if [ "${SPEC}" = "linux-g++-64" ]; then
-        wget https://s3-us-west-2.amazonaws.com/qgroundcontrol/dependencies/Qt5.8.0-linux-min.tar.bz2 &&
-        tar jxf Qt5.8.0-linux-min.tar.bz2 -C /tmp &&
-        export PATH=/tmp/Qt5.8-linux/5.8/gcc_64/bin:$PATH &&
+        wget https://s3-us-west-2.amazonaws.com/qgroundcontrol/dependencies/Qt5.7.1-linux-min.tar.bz2 &&
+        tar jxf Qt5.7.1-linux-min.tar.bz2 -C /tmp &&
+        export PATH=/tmp/Qt5.7-linux/5.7/gcc_64/bin:$PATH &&
         export DISPLAY=:99.0 &&
         sh -e /etc/init.d/xvfb start
         ;