Need to use a concurrent dictionary here otherwise we get errors

This is the error:
System.NullReferenceException was caught
  HResult=-2147467261
  Message=Object reference not set to an instance of an object.
  Source=mscorlib
  StackTrace:
       at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
       at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
       at ripple.Nuget.Cache`2.set_Item(TKey key, TValue value) in c:\Projects\ripple\src\ripple\Nuget\NugetFeedBase.cs:line 99
       at ripple.Nuget.NugetFeedBase.FindLatest(Dependency query) in c:\Projects\ripple\src\ripple\Nuget\NugetFeedBase.cs:line 268
  InnerException:
Ensuring we check the local cache for float version