Restrict DependencyMap#Get to reference types

It's impossible to add non-reference types to the map, so why allow pulling them out of it.

src/Discord.Net.Commands/Dependencies/DependencyMap.cs

@@ -57,7 +57,7 @@ namespace Discord.Commands
         }
 
         /// <inheritdoc />
-        public T Get<T>()
+        public T Get<T>() where T : class
         {
             return (T)Get(typeof(T));
         }
@@ -72,7 +72,7 @@ namespace Discord.Commands
         }
 
         /// <inheritdoc />
-        public bool TryGet<T>(out T result)
+        public bool TryGet<T>(out T result) where T : class
         {
             object untypedResult;
             if (TryGet(typeof(T), out untypedResult))

src/Discord.Net.Commands/Dependencies/IDependencyMap.cs

@@ -63,14 +63,14 @@ namespace Discord.Commands
         /// </summary>
         /// <typeparam name="T">The type of service.</typeparam>
         /// <returns>An instance of this service.</returns>
-        T Get<T>();
+        T Get<T>() where T : class;
         /// <summary>
         /// Try to pull an object from the map.
         /// </summary>
         /// <typeparam name="T">The type of service.</typeparam>
         /// <param name="result">The instance of this service.</param>
         /// <returns>Whether or not this object could be found in the map.</returns>
-        bool TryGet<T>(out T result);
+        bool TryGet<T>(out T result) where T : class;
 
         /// <summary>
         /// Pull an object from the map.