Bump System.Net.Http from 2.0.20710.0 to 4.3.4 in /sdk

Bumps System.Net.Http from 2.0.20710.0 to 4.3.4.

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

sdk/packages.config

@@ -3,5 +3,5 @@
   <package id="Google.Protobuf.Tools" version="3.6.1" targetFramework="net40" />
   <package id="Google.ProtocolBuffers" version="2.4.1.555" targetFramework="net45" />
   <package id="Microsoft.Net.Http" version="2.0.20710.0" targetFramework="net40" />
-  <package id="System.Net.Http" version="2.0.20710.0" targetFramework="net40" />
+  <package id="System.Net.Http" version="4.3.4" targetFramework="net40" />
 </packages>