chore: restrict Travis' matching of release tags

It used to also match "within" the branch name, which was not the intended behavior.

Author: RomainMuller

.travis.yml

@@ -3,7 +3,7 @@ language: minimal
 branches:
   only:
     - master
-    - /v\d+\.\d+.\d+/
+    - /^v\d+\.\d+.\d+$/
 
 cache:
   yarn: true