Update ClientRuntime version to latest

[cormacpayne]

Description

Update Microsoft.Rest.ClientRuntime to version 2.3.8 and Microsoft.Rest.ClientRuntime.Azure to version 3.3.8

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This checklist is used to make sure that common guidelines for a pull request are followed. You can find a more complete discussion of PowerShell cmdlet best practices here.

• I have read the contribution guidelines.
• If changes were made to any cmdlet, the XML help was regenerated using the platyPSHelp module.
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[markcowl]

on demand run here: http://azuresdkci.cloudapp.net/view/1-AzurePowerShell/job/powershell-demand/1819/