Use pipefail in travis

.travis.yml

@@ -14,5 +14,5 @@ install:
   - touch data/world_boundaries/builtup_area.shp data/simplified-land-polygons-complete-3857/simplified_land_polygons.shp data/ne_110m_admin_0_boundary_lines_land/ne_110m_admin_0_boundary_lines_land.shp data/ne_10m_populated_places/ne_10m_populated_places_fixed.shp data/land-polygons-split-3857/land_polygons.shp
 script:
   - jsonlint project.mml
-  - ./node_modules/carto/bin/carto project.mml | xmllint - | wc -l
+  - set -o pipefail && ./node_modules/carto/bin/carto project.mml | xmllint - | wc -l
   - ./scripts/travis_check_project_files