try fix osx build on Travis (go-sql-driver#994)

methane · tz70s · commit 8ce367163efd · 2020-09-05T10:04:50.000+08:00
diff --git a/.travis.yml b/.travis.yml
@@ -105,6 +105,7 @@ matrix:
         homebrew:
           packages:
             - mysql
+          update: true
       go: 1.12.x
       before_install:
         - go get golang.org/x/tools/cmd/cover
