Travis: Use travis_wait for installing Qt.

- Sometimes installing Qt takes more than 10 minutes and since it has not output we have to use travis_wait
- For more info see: https://docs.travis-ci.com/user/common-build-problems/#Build-times-out-because-no-output-was-received
- Also note that travis_wait will output to a file so its output will not mixed with job output:
- see https://github.com/travis-ci/travis-ci/issues/5071

.travis.yml

@@ -41,7 +41,7 @@ install:
               hdiutil attach -noverify $QT_INSTALLER_FILE_NAME.dmg;
               QT_INSTALLER=/Volumes/$QT_INSTALLER_FILE_NAME/$QT_INSTALLER_FILE_NAME.app/Contents/MacOS/$QT_INSTALLER_FILE_NAME;
               echo "Silently installing Qt...";
-              $QT_INSTALLER --script $QZ_DIR/mac/qt-mac-silent-install.qs;
+              travis_wait $QT_INSTALLER --script $QZ_DIR/mac/qt-mac-silent-install.qs;
          fi
     fi