Upgrade to TimeZoneInfo usage so that all builds work and are better.

Microsoft recommends to use TimeZoneInfo instead of TimeZone for all recent
versions (starting from 3.5) of the .NET, plus this is the only class
available in non-mainstream flavors of .NET (like Silverlight),
so why not use it.
Also update the assembly version of the SL5 build of ServiceStack.Text.

Author: ddotlic

src/ServiceStack.Text.SL5/Properties/AssemblyInfo.cs

@@ -21,4 +21,4 @@
 
 // The following GUID is for the ID of the typelib if this project is exposed to COM
 [assembly: Guid("b058736d-b210-47e1-b16f-ba72d92e7c4e")]
-[assembly: AssemblyVersion("3.9.44.0")]
+[assembly: AssemblyVersion("3.9.48.0")]

src/ServiceStack.Text/DateTimeExtensions.cs

@@ -41,7 +41,7 @@ public static long ToUnixTimeMsAlt(this DateTime dateTime)
             return (dateTime.ToStableUniversalTime().Ticks - UnixEpoch) / TimeSpan.TicksPerMillisecond;
         }
 
-        private static TimeZone LocalTimeZone = TimeZone.CurrentTimeZone;
+        private static TimeZoneInfo LocalTimeZone = TimeZoneInfo.Local;
         public static long ToUnixTimeMs(this DateTime dateTime)
         {
             var dtUtc = dateTime;