Travis CI builds should be successful, even if they fail for OSX

According to
https://docs.travis-ci.com/user/multi-os/
it should be

matrix:
  allow_failures:
    - os: osx

instead of

env:
  matrix:
    allow_failures:
      - os: osx

However, the page also states
"The feature described in this document is considered beta. Some features may not work as described."

.travis.yml

@@ -2326,6 +2326,8 @@ env:
 #      ENABLE_WEBSOCKETS=YES
 #      ENABLE_LUA=YES
 #      ENABLE_DUKTAPE=YES
+
+matrix:
   allow_failures:
     - os: osx