Bail out if any Travis CI testing commands fail

Previously we could fail some "mvn" stage but keep running anyway,
thereby fooling us into thinking that everything was OK.

.travis.yml

@@ -7,7 +7,9 @@ addons:
     packages:
     - realpath
     - xvfb
-before_install: source "travis/before-install-$BUILD_SYSTEM"
+before_install:
+  - set -e
+  - source "travis/before-install-$BUILD_SYSTEM"
 install: source "travis/install-$BUILD_SYSTEM"
 script: source "travis/script-$BUILD_SYSTEM"
 before_cache: source "travis/before-cache-$BUILD_SYSTEM"