TravisCI: use stages to end run when newest compilers fail

These tend to have the best error messages.

.travis.yml

@@ -13,6 +13,11 @@ cache:
   directories:
     - ${TRAVIS_BUILD_DIR}/deps
 
+stages:
+  - name: Latest     # build jobs to run first and always
+  - name: Validation # run other jobs
+  - name: Legacy     # build with all other supported compilers
+
 matrix:
   include:
 
@@ -20,7 +25,8 @@ matrix:
     # Validate CMake configuration
     ##########################################################################
 
-    - name: CMake validation on Linux
+    - name: CMake 3.1.3 - latest
+      stage: Validation
       env: &CMAKE_VERSION_LIST
       - CMAKE_VERSION: '"3.16.2 3.15.6 3.14.7 3.13.5 3.12.4 3.11.4 3.10.3 3.9.6 3.8.2 3.7.2 3.6.3 3.5.2 3.4.3 3.3.2 3.2.3 3.1.3"'
       - GSL_CXX_STANDARD: 14
@@ -41,7 +47,8 @@ matrix:
           for CMAKE in ${CMAKE_path[@]}; do test_CMake_generate $CMAKE; done
         )
 
-    - name: CMake validation on OSX
+    - name: CMake 3.2.3 - 3.16.2
+      stage: Validation
       os: osx
       osx_image: xcode11.3
       env:
@@ -59,6 +66,7 @@ matrix:
 
     # XCode 8.3
     - name: AppleClang Xcode-8.3 C++14 Debug
+      stage: Legacy
       env: BUILD_TYPE=Debug GSL_CXX_STANDARD=14
       os: osx
       osx_image: xcode8.3
@@ -141,6 +149,7 @@ matrix:
 
     # XCode 11.3
     - name: AppleClang Xcode-11.3 C++17 Debug
+      stage: Latest
       env: BUILD_TYPE=Debug GSL_CXX_STANDARD=17
       os: osx
       osx_image: xcode11.3 # AppleClang 11.0.0 linker update / same as Xcode 11.2
@@ -163,6 +172,7 @@ matrix:
 
     # Clang 3.6
     - name: Clang-3.6 C++14 Debug
+      stage: Legacy
       dist: xenial
       env: CXX=clang++-3.6 BUILD_TYPE=Debug GSL_CXX_STANDARD=14
       addons: &clang36
@@ -288,6 +298,7 @@ matrix:
 
     # Clang 9
     - name: Clang-9 C++14 Debug
+      stage: Latest
       env: CXX=clang++-9 BUILD_TYPE=Debug GSL_CXX_STANDARD=14
       addons: &clang9
         apt:
@@ -312,6 +323,7 @@ matrix:
 
     # GCC 5 (default on the Xenial image)
     - name: GCC-5 C++14 Debug
+      stage: Legacy
       dist: xenial
       env: CXX=g++-5 BUILD_TYPE=Debug GSL_CXX_STANDARD=14
     - name: GCC-5 C++14 Release
@@ -356,6 +368,7 @@ matrix:
 
     # GCC 9
     - name: GCC-9 C++14 Debug
+      stage: Latest
       env: CXX=g++-9 BUILD_TYPE=Debug GSL_CXX_STANDARD=14
       addons: &gcc9
         apt: