Minimize the set of excluded jobs.

.travis.yml

@@ -12,10 +12,6 @@ env:
   global:
   - HOMEBREW_NO_AUTO_UPDATE=1 # workaround for #421
   matrix:
-  - CHECK_CLANG_FORMAT=1
-  - DOCKER_BUILD=ubuntu-16.04
-  - DOCKER_BUILD=ubuntu-18.04
-  - DOCKER_BUILD=ubuntu-19.10
   - LLVM_CONFIG=llvm-config-3.5 CLANG=clang-3.5
   - LLVM_CONFIG=llvm-config-3.8 CLANG=clang-3.8
   - LLVM_CONFIG=llvm-config-3.8 CLANG=clang-3.8 USE_CMAKE=1
@@ -33,34 +29,20 @@ env:
   - LLVM_CONFIG=llvm-config-9 CLANG=clang-9 USE_CMAKE=1 STATIC_LLVM=0 STATIC_LUAJIT=0
   - LLVM_CONFIG=llvm-config-9 CLANG=clang-9 USE_CMAKE=1 SLIB_INCLUDE_LLVM=0 SLIB_INCLUDE_LUAJIT=0 USE_CUDA=1
 
-matrix:
-  exclude:
-    # Only need one copy of this test.
+jobs:
+  include:
     - os: linux
-      compiler: clang
       env: CHECK_CLANG_FORMAT=1
     - os: linux
-      compiler: clang
       env: DOCKER_BUILD=ubuntu-16.04
     - os: linux
-      compiler: clang
       env: DOCKER_BUILD=ubuntu-18.04
     - os: linux
-      compiler: clang
       env: DOCKER_BUILD=ubuntu-19.10
+  exclude:
     # Various macOS tests seem to be broken.
     - os: osx
       compiler: gcc
-    - os: osx
-      env: CHECK_CLANG_FORMAT=1
-    - os: osx
-      env: DOCKER_BUILD=ubuntu-16.04
-    - os: osx
-      env: DOCKER_BUILD=ubuntu-18.04
-    - os: osx
-      env: DOCKER_BUILD=ubuntu-19.10
-    - os: osx
-      env: LLVM_CONFIG=llvm-config-3.8 CLANG=clang-3.8
     - os: osx
       env: LLVM_CONFIG=llvm-config-5.0 CLANG=clang-5.0 USE_CMAKE=1
     - os: osx