Fixed tag build trigger

Our version tags are not using the `v` prrefix, this assures that
when a tag is pushed, we trigger a build.

Deploy stage is already configured to trigger only on tags.

Change-Id: Id495b012bb37312d5fc02e25ed19272e77eaebc2

Author: ssbarnea

.travis.yml

@@ -9,7 +9,7 @@ language: python
 branches:
   only:
     - master
-    - /v[0-9]+\.[0-9]+\.[0-9]+(-.*)?$/
+    - /^\d+\.\d+(\.\d+)?(-\S*)?$/
 
 cache:
   - pip