Fix RootNamespace, DefaultLanguage, and old package refs


Before this PR, I see the following warnings when building in VS 2013:

```
Warning	1	Load of property 'RootNamespace' failed.  The string for the root namespace must be a valid identifier.	Octokit.Tests-NetCore45
Warning	2	Some NuGet packages were installed using a target framework different from the current target framework and may need to be reinstalled. For more information, visit http://docs.nuget.org/docs/workflows/reinstalling-packages.  Packages affected: Rx-Core, Rx-Interfaces, Rx-Linq, Rx-PlatformServices		0	0	Octokit
Warning	3	Some NuGet packages were installed using a target framework different from the current target framework and may need to be reinstalled. For more information, visit http://docs.nuget.org/docs/workflows/reinstalling-packages.  Packages affected: Rx-Core, Rx-Interfaces, Rx-Linq, Rx-PlatformServices		0	0	Octokit.Reactive
Warning	4	The DefaultLanguage property is either missing from the project file or does not have a value. The fallback language is set to the Visual Studio language: en-AU.	Octokit-NetCore45
Warning	5	The project 'Octokit-NetCore45' cannot be referenced. The referenced project is targeted to a different framework family (.NETCore)	Octokit.Tests-NetCore45
```

After this PR, I see the following warnings (I don't want to attempt to address these in the same PR):
```
Warning	1	Some NuGet packages were installed using a target framework different from the current target framework and may need to be reinstalled. For more information, visit http://docs.nuget.org/docs/workflows/reinstalling-packages.  Packages affected: Rx-Core, Rx-Interfaces, Rx-Linq, Rx-PlatformServices		0	0	Octokit.Reactive
Warning	2	The project 'Octokit-NetCore45' cannot be referenced. The referenced project is targeted to a different framework family (.NETCore)	Octokit.Tests-NetCore45
```