Remove OSX + Python 3.7 & nightly

It is unsupported and causes problems. See [here](hardbyte#554 (comment)).

Author: felixdivo

.travis.yml

@@ -63,14 +63,6 @@ jobs:
       os: osx
       osx_image: xcode8.3
       python: 3.6-dev
-    - stage: test
-      os: osx
-      osx_image: xcode8.3
-      python: 3.7-dev
-    - stage: test
-      os: osx
-      osx_image: xcode8.3
-      python: nightly
 
     - stage: documentation
       name: "Sphinx Build"