Rebase onto .NET Core 1.1 (#2737)

* Rebase onto .NET Core 1.1

Modify `Build.psm1` and `project.json` files to use .NET Core 1.1.

.NET Core 1.1 ships with an older dotnet/cli than has currently been used,
so we revert to use case-sensitive directory names for dependencies.

.NET Core 1.1 is a pre-requisite for supporting Fedora 24.

* PSReadLine: Bump major version of PSReadline to 6.0.0-*

* Microsoft.PowerShell.Commands.Utility: Bump major version of Microsoft.CodeAnalysis.Csharp

* Microsoft.PowerShell.SDK: Revert attempt to import netcoreapp1.1 instead of dnxcore

* build.psm1: Restore ability to specify version of dotnet-install.sh

docs/building/linux.md

@@ -101,7 +101,7 @@ Start-PSBuild
 Congratulations! If everything went right, PowerShell is now built.
 The `Start-PSBuild` script will output the location of the executable:
 
-`./src/powershell-unix/bin/Linux/netcoreapp1.0/ubuntu.14.04-x64/powershell`.
+`./src/powershell-unix/bin/Linux/netcoreapp1.1/ubuntu.14.04-x64/powershell`.
 
 You should now be running the `powershell` that you just built, if your run the above executable.
 You can run our cross-platform Pester tests with `Start-PSPester`, and our xUnit tests with `Start-PSxUnit`.

docs/building/macos.md

@@ -68,6 +68,6 @@ download the `pkg` from our GitHub releases page using your browser, complete th
 start a `powershell` session, and use `Start-PSBuild` from the module.
 
 The output directory will be slightly different because your runtime identifier is different.
-PowerShell will be at `./src/powershell-unix/bin/Linux/netcoreapp1.0/osx.10.11-x64/powershell`,
+PowerShell will be at `./src/powershell-unix/bin/Linux/netcoreapp1.1/osx.10.11-x64/powershell`,
 or `osx.10.10` depending on your operating system version.
 Note that configuration is still `Linux` because it would be silly to make yet another separate configuration when it's used solely to work-around a CLI issue.

docs/building/windows-core.md

@@ -64,7 +64,7 @@ Congratulations! If everything went right, PowerShell is now built and executabl
 
 This location is of the form `./[project]/bin/[configuration]/[framework]/[rid]/[binary name]`, 
 and our project is `powershell`, configuration is `Debug` by default, 
-framework is `netcoreapp1.0`, runtime identifier is **probably** `win10-x64` 
+framework is `netcoreapp1.1`, runtime identifier is **probably** `win10-x64` 
 (but will depend on your operating system; 
 don't worry, `dotnet --info` will tell you what it was), and binary name is `powershell`. 
 The function `Get-PSOutput` will return the path to the executable;