Attempt to get x-plat builds running

.travis.yml cobbled together from samples at

https://docs.microsoft.com/en-us/dotnet/articles/core/tools/using-ci-with-cli

.travis.yml

@@ -1,14 +1,22 @@
 language: csharp
-sudo: required
-dist: trusty
-dotnet: 1.0.0-preview2-003121
-mono: none
 solution: ImageSharp.sln
 
-branches:  
+matrix:
+  include:
+    - os: linux # Ubuntu 14.04
+      dist: trusty
+      sudo: required
+      dotnet: 1.0.0-preview2-003121
+      mono: latest
+    - os: osx # OSX 10.11
+      osx_image: xcode7.2
+      dotnet: 1.0.0-preview2-003121
+      mono: latest
+
+branches:
   only:
     - master
-    
+
 script:
   - cd src/ImageSharp/
   - dotnet restore

NuGet.config

@@ -1,7 +1,8 @@
 <?xml version="1.0" encoding="utf-8"?>
 <configuration>
   <packageSources>
+    <add key="myget.org dotnet-core" value="https://dotnet.myget.org/F/dotnet-core/api/v3/index.json" />
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
-    <add key="benchmarkDotNetDevelop" value="https://ci.appveyor.com/nuget/benchmarkdotnet" />
+    <add key="nuget.org" value="https://www.nuget.org/api/v2/" />
   </packageSources>
 </configuration>

src/ImageSharp/project.json

@@ -64,6 +64,9 @@
       }
     },
     "net45": {
+      "dependencies": {
+        "System.Runtime": "4.0.0"
+      }
     }
   }
 }