Add Visual Studio 2015 into CI

[JacksonTian]

You need to complete

• [x] unit tests and/or feature tests
• [ ] documentation is changed or added

[codecov-io]

Codecov Report

Merging #185 into master will decrease coverage by 0.03%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #185      +/-   ##
==========================================
- Coverage   92.66%   92.63%   -0.04%     
==========================================
  Files          62       62              
  Lines        2960     2960              
  Branches      280      280              
==========================================
- Hits         2743     2742       -1     
  Misses        173      173              
- Partials       44       45       +1

Flag	Coverage Δ
#unittest	92.63% <ø> (-0.04%)	:arrow_down:

Impacted Files	Coverage Δ
...dk-core/Auth/InstanceProfileCredentialsProvider.cs	97.22% <0%> (-2.78%)	:arrow_down:

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[JacksonTian]

竟然 CI 没有出错。

[AxiosLeo]

竟然 CI 没有出错。

人品好啊！