Travis: disable failing qt56 build

For some reason openSUSE 42.3 updates keep timing out, causing these builds to
fail. Having a Qt5.5 and a Qt5.7 build should be sufficient for our purposes.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

.travis.yml

@@ -136,12 +136,12 @@ matrix:
       services:
           - docker
 
-    - env: SUBSURFACE_PLATFORM="qt56"
+#    - env: SUBSURFACE_PLATFORM="qt56"
-      os: linux
+#      os: linux
-      language: c++
+#      language: c++
-      filter_secrets: false
+#      filter_secrets: false
-      services:
+#      services:
-          - docker
+#          - docker
 
     - env: SUBSURFACE_PLATFORM="qt57"
       os: linux