PROTON-2403, PROTON-2351: remove 'allow fail' on Travis MacOS jobs now that 471d043db648e9fc855ae2d9330fa5dfb4332dd3 changes got things working again

This reverts commit cbdb8d531ade5b4bb9d037d3383ac0ddd81613aa.
diff --git a/.travis.yml b/.travis.yml
index c9681e0..fae492b 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -18,8 +18,6 @@
 #
 
 jobs:
-  allow_failures:
-  - os: osx
   include:
   - os: linux
     dist: xenial