[CI] enable slack notifications

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 3d3e211..ff79220 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -27,6 +27,9 @@
 os:
   - linux
 
+notifications:
+  slack: efabless-team:XAEJOuCet9xQ14EEIO0nNufd
+
 jobs:
   include:
     - name: "The Precheck Test"