ci: trim down Appveyor

.appveyor.yml

 version: 1.0.{build}
 image:
-- Visual Studio 2017
 - Visual Studio 2015
 test: off
 skip_branch_with_pr: true
 build:
   parallel: true
 platform:
-- x64
 - x86
 environment:
   matrix:
   - PYTHON: 36
-    CPP: 14
     CONFIG: Debug
   - PYTHON: 27
-    CPP: 14
     CONFIG: Debug
-  - CONDA: 36
-    CPP: latest
-    CONFIG: Release
-matrix:
-  exclude:
-    - image: Visual Studio 2015
-      platform: x86
-    - image: Visual Studio 2015
-      CPP: latest
-    - image: Visual Studio 2017
-      CPP: latest
-      platform: x86
 install:
 - ps: |
-    if ($env:PLATFORM -eq "x64") { $env:CMAKE_ARCH = "x64" }
-    if ($env:APPVEYOR_JOB_NAME -like "*Visual Studio 2017*") {
-      $env:CMAKE_GENERATOR = "Visual Studio 15 2017"
-      $env:CMAKE_INCLUDE_PATH = "C:\Libraries\boost_1_64_0"
-      $env:CXXFLAGS = "-permissive-"
-    } else {
     $env:CMAKE_GENERATOR = "Visual Studio 14 2015"
-    }
-    if ($env:PYTHON) {
     if ($env:PLATFORM -eq "x64") { $env:PYTHON = "$env:PYTHON-x64" }
     $env:PATH = "C:\Python$env:PYTHON\;C:\Python$env:PYTHON\Scripts\;$env:PATH"
     python -W ignore -m pip install --upgrade pip wheel
     python -W ignore -m pip install pytest numpy --no-warn-script-location
-    } elseif ($env:CONDA) {
-      if ($env:CONDA -eq "27") { $env:CONDA = "" }
-      if ($env:PLATFORM -eq "x64") { $env:CONDA = "$env:CONDA-x64" }
-      $env:PATH = "C:\Miniconda$env:CONDA\;C:\Miniconda$env:CONDA\Scripts\;$env:PATH"
-      $env:PYTHONHOME = "C:\Miniconda$env:CONDA"
-      conda --version
-      conda install -y -q pytest numpy scipy
-    }
 - ps: |
     Start-FileDownload 'http://bitbucket.org/eigen/eigen/get/3.3.3.zip'
     7z x 3.3.3.zip -y > $null
     $env:CMAKE_INCLUDE_PATH = "eigen-eigen-67e894c6cd8f;$env:CMAKE_INCLUDE_PATH"
 build_script:
 - cmake -G "%CMAKE_GENERATOR%" -A "%CMAKE_ARCH%"
-    -DPYBIND11_CPP_STANDARD=/std:c++%CPP%
+    -DPYBIND11_CPP_STANDARD=/std:c++14
     -DPYBIND11_WERROR=ON
     -DDOWNLOAD_CATCH=ON
     -DCMAKE_SUPPRESS_REGENERATION=1
@@ -66,5 +34,4 @@ build_script:
 - set MSBuildLogger="C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
 - cmake --build . --config %CONFIG% --target pytest -- /m /v:m /logger:%MSBuildLogger%
 - cmake --build . --config %CONFIG% --target cpptest -- /m /v:m /logger:%MSBuildLogger%
-- if "%CPP%"=="latest" (cmake --build . --config %CONFIG% --target test_cmake_build -- /m /v:m /logger:%MSBuildLogger%)
 on_failure: if exist "tests\test_cmake_build" type tests\test_cmake_build\*.log*