paket pack fails when semver contains number + build metadata only #704

$ paket pack output foo version 1.0.0+foobar -v
Paket version 0.35.0.0
found: blah\paket.dependencies
Paket failed with:
        Input string was not in a correct format.
StackTrace:
     at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal)
   at System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info)
   at Paket.SemVer.Parse(String version)
   at Microsoft.FSharp.Core.OptionModule.Map[T,TResult](FSharpFunc`2 mapping, FSharpOption`1 option)
   at Paket.PackageProcess.Pack(DependenciesFile dependencies, String packageOutputPath, FSharpOption`1 buildConfig, FSharpOption`1 version, FSharpOption`1 releaseNotes)
   at Paket.Program.clo@198-21.Invoke(ArgParseResults`1 results)
   at Paket.Program.processWithValidation[T](FSharpFunc`2 validateF, FSharpFunc`2 commandF, Command command, String[] args)
   at Paket.Program.processCommand@59-1.Invoke(Command command, String[] args)
   at <StartupCode$paket>.$Paket.Program.main@()

$ paket pack output foo version 1.0.0-beta+foobar
Paket version 0.35.0.0
found: blah\paket.dependencies
1 second - ready.