chore(travis): fix ROOT_DIR path when build.sh is sourced

Author: gkalpak

scripts/travis/build.sh

@@ -2,7 +2,7 @@
 
 set -e
 
-readonly THIS_DIR=$(cd $(dirname $0); pwd)
+readonly THIS_DIR=$(cd $(dirname ${BASH_SOURCE[0]}); pwd)
 readonly ROOT_DIR="$THIS_DIR/../.."
 
 export BROWSER_STACK_ACCESS_KEY