Use shell=True as workaround for windows bugs
Use shell=True as workaround for windows bugs

Letting sarge handle quoting the scratch org alias via shell_format
works on macOS and Linux, but fails when tested in Windows 10:

```powershell
(cci) PS C:\Users\IEUser\Documents\GitHub\CumulusCI-Test> cci org info itchy1
[32m2018-07-30 18:08:14:[0m Creating scratch org with command sfdx force:org:create -f orgs/dev.json -n --durationdays 7 -a 'CumulusCI Test__itchy1'
ERROR:  Setting variables must be in the format <key>=<value> or <key>="<value with spaces>" but found Test__itchy1'.
Error: ScratchOrgException: Failed to create scratch org:
```

i.e.: `\'CumulusCI Test__itchy\'`

SalesforceFoundation/CumulusCI,SalesforceFoundation/CumulusCI