[travis] use depreciated Trusty images

- see also: https://blog.travis-ci.com/2017-06-21-trusty-updates-2017-Q2-launch
- we may check this until September 1st

.travis.yml

@@ -21,3 +21,5 @@ install:
 script:
   - docker run -v $PWD:/build calamares bash -lc "cd /build && cmake -DWEBVIEW_FORCE_WEBKIT=1 -DKDE_INSTALL_USE_QT_SYS_PATHS=ON . && make -j2"
   - pycodestyle --exclude=thirdparty,.git $PWD
+
+group: deprecated-2017Q2