fix nuget upload to nuget.org (#136)

* fix nuspec

upload is failing with the following: 

```
Response status code does not indicate success: 400 (The readme file 'readme.md' does not exist in the package.).
```

* fix project url

Author: stevenbrix

nuget/swiftwinrt.nuspec

@@ -11,10 +11,10 @@
     <releaseNotes></releaseNotes>
     <tags>swift WinRT</tags>
     <copyright>© The Browser Company of New York. All rights reserved.</copyright>
-    <projectUrl>https://github.com/thebrowsercompany/swiftwinrt</projectUrl>
-    <repository type="git" url="https://github.com/thebrowsercompany/swiftwinrt"/>
+    <projectUrl>https://github.com/thebrowsercompany/swift-winrt</projectUrl>
+    <repository type="git" url="https://github.com/thebrowsercompany/swift-winrt"/>
     <license type="expression">BSD-3-Clause</license>
-    <readme>readme.md</readme>
+    <readme>README.md</readme>
   </metadata>
   <files>
     <file src="$swiftwinrt_exe$" target="bin"/>