DRi#2764 Travis OSX: skip OSX for PR's (#2074)

Skips running OSX for pull requests for now, since the Travis queue is
too long. We'll rely on the push to master to catch OSX issues and the
dev will have to revert or fix quickly at that point.

Re-writes the Travis build config from a matrix defined by os, compiler,
and env with exclusions to an explicit list of jobs, in order to use
conditional jobs. This is also clearer and easier to read.

.travis.yml

@@ -1,5 +1,5 @@
 # **********************************************************
-# Copyright (c) 2016-2017 Google, Inc.  All rights reserved.
+# Copyright (c) 2016-2018 Google, Inc.  All rights reserved.
 # **********************************************************
 
 # Dr. Memory: the memory debugger
@@ -41,20 +41,20 @@ language:
   - c
   - cpp
 
-os:
-  - linux
-  - osx
-
-# We request 2 runs, one with clang and one with gcc:
-compiler:
-  - clang
-  - gcc
-
-matrix:
-  exclude:
-    # gcc on Travis claims to not be CMAKE_COMPILER_IS_GNUCC so we disable:
-    - os: osx
+# We use a jobs include approach rather than an os, compiler, env matrix
+# with exludes so we can use conditional builds (plus it's clearer this way).
+jobs:
+  include:
+    - os: linux
       compiler: gcc
+    - os: linux
+      compiler: clang
+    - os: osx
+      # gcc on Travis claims to not be CMAKE_COMPILER_IS_GNUCC so we only run clang.
+      compiler: clang
+      # XXX: DRi#2764: Travis OSX resources are over-subscribed and it can take
+      # hours to get an OSX machine, so we skip running PR's for now.
+      if: type = push
 
 # For C/C++ there is no default install, so we set "install", not "before_install".
 install: