Create AppImage on Travis

Please wait until merged:

https://github.com/probonopd/AppImages/pull/241

Forum discussion:

https://forum.freecadweb.org/viewtopic.php?f=4&t=22606

.travis.yml

@@ -227,4 +227,7 @@ after_success:
      deployContext=$(create_helper_context repo=${TRAVIS_REPO_SLUG} auth_token=${GH_TOKEN} release=${DEPLOY_RELEASE})
      gitHub_deploy_asset_to_release_named $deployContext ${DEPLOYMENT_ARCHIVE}
      gitHub_prune_assets_for_release_named $deployContext "-${QT}" 1
+  elif [ "${TRAVIS_OS_NAME}" == "linux" -a "${TRAVIS_PULL_REQUEST}" == "false" ]; then
+     wget https://raw.githubusercontent.com/probonopd/AppImages/master/recipes/meta/Recipe
+     bash Recipe FreeCAD-nightly
   fi