[REEF-1954] Require Visual Studio 2017

This change

   * Updates `lang/cs/build.md` to require Visual Studio 2017
   * Updates `appveyor.yml` to only build on Visual Studio 2017
   * Updated the solutions file to require Visual Studio 2017

JIRA: [REEF-1954](https://issues.apache.org/jira/browse/REEF-1954)

Pull Request:
  This closes #1415
diff --git a/appveyor.yml b/appveyor.yml
index c882e23..e28c0b0 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -18,8 +18,6 @@
 shallow_clone: true
 
 os:
- - Visual Studio 2013
- - Visual Studio 2015
  - Visual Studio 2017
 
 platform: x64
diff --git a/lang/cs/BUILD.md b/lang/cs/BUILD.md
index dad75d5..00d4b17 100644
--- a/lang/cs/BUILD.md
+++ b/lang/cs/BUILD.md
@@ -34,9 +34,9 @@
   * Add the following items to the environment variable `PATH`:
      * Add the location of the `protoc.exe` executable in the windows path
      * Add `%JAVA_HOME%/bin` and `%M2_HOME%/bin` to the windows path as well
-  * [Visual Studio](http://www.visualstudio.com) 2015 (preferred) or 2013. Most REEF developers use the free Community
+  * [Visual Studio](http://www.visualstudio.com) 2017 V. 15.4.3 or newer. Most REEF developers use the free Community
     Edition.
-  * NuGet 2.8.6 or later. (Included in VS 2015)
+  * NuGet 4.0.0 or later. (Included in VS 2017)
   * xunit.runner.console.2.1.0 package (installing it might require manual restore of NuGet packages).
 
 
diff --git a/lang/cs/Org.Apache.REEF.sln b/lang/cs/Org.Apache.REEF.sln
index 3889f24..b4cbccd 100644
--- a/lang/cs/Org.Apache.REEF.sln
+++ b/lang/cs/Org.Apache.REEF.sln
Binary files differ