Update System.Runtime reference for NET4.5

ImageSharp.sln

@@ -11,6 +11,7 @@ Project("{8BB2217D-0F2D-49D1-97BC-3654ED321F3B}") = "ImageSharp.Benchmarks", "te
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "SolutionItems", "SolutionItems", "{C317F1B1-D75E-4C6D-83EB-80367343E0D7}"
 	ProjectSection(SolutionItems) = preProject
+		.travis.yml = .travis.yml
 		build\appveyor-project-version-patch.js = build\appveyor-project-version-patch.js
 		build\appveyor-semver.ps1 = build\appveyor-semver.ps1
 		appveyor.yml = appveyor.yml
@@ -22,7 +23,6 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "SolutionItems", "SolutionIt
 		build\package.json = build\package.json
 		README.md = README.md
 		Settings.StyleCop = Settings.StyleCop
-		travis.yml = travis.yml
 	EndProjectSection
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Source", "Source", "{815C0625-CD3D-440F-9F80-2D83856AB7AE}"

src/ImageSharp/project.json

@@ -67,7 +67,7 @@
       "dependencies": {
       },
       "frameworkAssemblies": {
-        "System.Runtime": ""
+        "System.Runtime": "4.0.0"
       }
     }
   }