Update SemVerHelper.fs

Changes for #522: SemVerHelper.PreRelease.TryParse cannot parse valid nuget pre-release identifiers
Make number string option
Change regex accordingly.
No tests found:
Tested via fsi.exe:
val x : PreRelease option = Some {Origin = "dev20140819130924";
                                  Name = "dev";
                                  Number = Some "20140819130924";}

> let x = PreRelease.TryParse("dev-2014-08-19T130924");;

val x : PreRelease option = Some {Origin = "dev-2014-08-19T130924";
                                  Name = "dev";
                                  Number = Some "-2014-08-19T130924";}