[appveyor] Add build jobs using VS2015 (#290) [travis skip]

.appveyor.yml

@@ -17,6 +17,18 @@ shallow_clone: true
 
 environment:
   matrix:
+    - TOOLSET: msvc-14.0
+      ARCH: x86_64
+      VARIANT: debug
+      CXXSTD: 11
+      TEST_HEADERS: 1
+      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
+    - TOOLSET: msvc-14.0
+      ARCH: x86_64
+      VARIANT: release
+      CXXSTD: 11
+      TEST_HEADERS: 1
+      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
     - TOOLSET: msvc-14.1
       ARCH: x86_64
       VARIANT: debug

README.md

@@ -14,8 +14,8 @@
 -----------|-----------------|-----------------|-----------------|-----------------|------------
 **develop**| [![AppVeyor](https://ci.appveyor.com/api/projects/status/w4k19d8io2af168h/branch/develop?svg=true)](https://ci.appveyor.com/project/stefanseefeld/gil/branch/develop) | [![Azure](https://dev.azure.com/boostorg/gil/_apis/build/status/boostorg.gil?branchName=develop)](https://dev.azure.com/boostorg/gil/_build/latest?definitionId=4?branchName=develop) | [![Travis](https://travis-ci.org/boostorg/gil.svg?branch=develop)](https://travis-ci.org/boostorg/gil) | [![CircleCI](https://circleci.com/gh/boostorg/gil/tree/develop.svg?style=shield)](https://circleci.com/gh/boostorg/workflows/gil/tree/develop) | [![gil](https://img.shields.io/badge/gil-develop-blue.svg)](http://www.boost.org/development/tests/develop/developer/gil.html)
 **master** | [![AppVeyor](https://ci.appveyor.com/api/projects/status/w4k19d8io2af168h?svg=true)](https://ci.appveyor.com/project/stefanseefeld/gil/branch/master) | [![Azure](https://dev.azure.com/boostorg/gil/_apis/build/status/boostorg.gil?branchName=master)](https://dev.azure.com/boostorg/gil/_build/latest?definitionId=4?branchName=master) | [![Travis](https://travis-ci.org/boostorg/gil.svg?branch=master)](https://travis-ci.org/boostorg/gil) | [![CircleCI](https://circleci.com/gh/boostorg/gil/tree/master.svg?style=shield)](https://circleci.com/gh/boostorg/workflows/gil/tree/master) | [![gil](https://img.shields.io/badge/gil-master-blue.svg)](http://www.boost.org/development/tests/master/developer/gil.html)
- **C++/compilers:** | 14, 17 | 11, 14, 17 | 11 | 11 |
- msvc++    | VS2017 | VS2017, VS2015 |   |   |
+ **C++/compilers:** | 11, 14, 17 | 11, 14, 17 | 11 | 11 |
+ msvc++    | VS2015, VS2017 | VS2015, VS2017 |   |   |
  clang++   |   | Xcode 9.4.1 | 3.9, 5.0, Xcode 9.4.1 | 3.9, 4.0, 5.0 |
  g++       |   | 5.4, 8.1 | 5.5, 6.5, 7.4 | 4.8-9, 5.1-5, 6.1-4, 7.1-3, 8.2 |
  **tests coverage:** |