From the package contents of the latest ServiceStack.Common
package there seems to be no -client version included (See Common Framework and Profile Targeting Examples under NuGet - Creating and Publishing a Package).
You should either change the target framework the full profile or pull the code from ServiceStack's GitHub Repo and try to build it for client profile yourself. The downside of this is that you will loose Nuget's awesome update features...
You could off course file an issue at GitHub for the guys maintaining ServiceStack to add a client profile version of the framework, but I guess that there's a good reason for why it's not there already.