build: use correct tag regex

Author: rokroskar

.travis.yml

@@ -50,7 +50,7 @@ stages:
   - name: publish 🐍
     if: type = push AND (branch = master OR tag IS present)
   - name: brew 🍺
-    if: type = push AND tag IS present AND tag =~ /^\d\.\d\.\d$/
+    if: type = push AND tag IS present AND tag =~ /^v\d\.\d\.\d$/
 
 before_install:
   - git fetch --tags