Remove macOS environments from build matrix

Travis' backlog for macOS builds is quite high, and the environments also do not support MongoDB. We can revisit adding these environments down the line when the infrastructure has improved and we sort out explicit installation of MongoDB versions (see: PHPLIB-299).

Note: a macOS build with Secure Channel was manually tested in mongodb#736.

Author: jmikola

.travis.yml

@@ -28,22 +28,6 @@ matrix:
       dist: trusty
       sudo: required
       php: 7.2
-    - os: osx
-      osx_image: xcode8.3
-      language: generic
-      env:
-        - PHP_VERSION: php56
-    - os: osx
-      osx_image: xcode9.2
-      language: generic
-      env:
-        - PHP_VERSION: php72
-    - os: osx
-      osx_image: xcode9.2
-      language: generic
-      env:
-        - PHP_VERSION: php72
-        - SSL_VERSION: darwin
 
 services:
   - mongodb