ci: Travis: add baseline stage [skip appveyor] (#10226)

This moves the 4 fastest jobs there, effectively resulting in a separate
OSX stage then.

Travis typically runs 4/5 jobs in parallel, so this avoids a) running the
slower OSX builds if there are problems already, and b) will start other
builds already earlier (e.g. after the lint job finished).

.travis.yml

@@ -49,7 +49,7 @@ env:
 
 jobs:
   include:
-    - stage: normal builds
+    - stage: baseline builds
       os: linux
       compiler: clang-4.0
       # Use Lua so that ASAN can test our embedded Lua support. 8fec4d53d0f6
@@ -68,15 +68,18 @@ jobs:
       # store 32-bit dependencies in a separate cache.
       compiler: gcc
       env: BUILD_32BIT=ON
-    - os: osx
+    - if: branch = master
+      os: linux
+      env: CI_TARGET=lint
+
+    - stage: OSX builds
+      os: osx
       compiler: clang
       osx_image: xcode10.1  # macOS 10.13
     - os: osx
       compiler: gcc
       osx_image: xcode10.1  # macOS 10.13
-    - if: branch = master
-      os: linux
-      env: CI_TARGET=lint
+
     - stage: Flaky builds
       os: linux
       compiler: gcc