build: open VC15 projects with VS 2017

Previously, they were opened with Visual Studio 2015 by default, which
cannot build them.

projects/Windows/VC15/curl-all.sln

@@ -1,5 +1,5 @@
 Microsoft Visual Studio Solution File, Format Version 12.00
-# Visual Studio 2015
+# Visual Studio 15
 Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "curl", "src\curl.vcxproj", "{5228E9CE-A216-422F-A5E6-58E95E2DD71D}"
 	ProjectSection(ProjectDependencies) = postProject
 		{DA6F56B4-06A4-441D-AD70-AC5A7D51FADB} = {DA6F56B4-06A4-441D-AD70-AC5A7D51FADB}

projects/Windows/VC15/lib/libcurl.sln

@@ -1,5 +1,5 @@
 Microsoft Visual Studio Solution File, Format Version 12.00
-# Visual Studio 2015
+# Visual Studio 15
 Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "libcurl", "libcurl.vcxproj", "{DA6F56B4-06A4-441D-AD70-AC5A7D51FADB}"
 EndProject
 Global

projects/Windows/VC15/src/curl.sln

@@ -1,5 +1,5 @@
 Microsoft Visual Studio Solution File, Format Version 12.00
-# Visual Studio 2015
+# Visual Studio 15
 Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "curl", "curl.vcxproj", "{5228E9CE-A216-422F-A5E6-58E95E2DD71D}"
 EndProject
 Global