Update .travis.yml

Adds regular expression to match tags (yes, match tag in branches...)
Tags should follow:

1.0.0
or
1.0.0-some

Author: flovilmart

.travis.yml

@@ -10,6 +10,7 @@ env:
 branches:
   only:
   - master
+  - /^[0-9].[0-9].[0-9](-.*)?$/
 cache:
   directories:
   - "$HOME/.mongodb/versions/downloads"