Enable caching of dotnet and NuGet packages on CI

.travis.yml

@@ -7,6 +7,10 @@ matrix:
     - os: osx
       osx_image: xcode8
       language: generic
+cache:
+  directories:
+    - $HOME/.dotnet
+    - $HOME/.nuget
 addons:
   artifacts:
     s3_region: "us-west-2"

appveyor.yml

@@ -1,5 +1,8 @@
 version: 0.5.0.{build}
 
+cache:
+  - '%LocalAppData%\Microsoft\dotnet'
+
 nuget:
   project_feed: true