Use set -e in Travis scripts to ensure the scripts fail on error

Tools/travis-linux-install.sh

@@ -1,4 +1,7 @@
 #!/bin/bash
+
+set -e # exit on failure
+
 lsb_release -a
 sudo apt-get -qq update
 

Tools/travis-osx-install.sh

@@ -1,4 +1,7 @@
 #!/bin/bash
+
+set -e # exit on failure
+
 sw_vers
 brew update
 brew list