Cannot use profiles on windows

When using the `default` profile, the shell works, but trying to specify a profile like `aws-shell --profile dev` results in:

```
Creating doc index in the background. It will be a few minutes before all documentation is available.
aws> ec2 describe-instances
Traceback (most recent call last):
File "c:\python27\lib\runpy.py", line 162, in _run_module_as_main
"__main__", fname, loader, pkg_name)
File "c:\python27\lib\runpy.py", line 72, in _run_code
exec code in run_globals
File "C:\Python27\Scripts\aws-shell.exe\__main__.py", line 9, in
File "c:\python27\lib\site-packages\awsshell\__init__.py", line 70, in main
shell.run()
File "c:\python27\lib\site-packages\awsshell\app.py", line 319, in run
p = self._popen_cls(full_cmd, shell=True, env=self._env)
File "c:\python27\lib\subprocess.py", line 710, in __init__
errread, errwrite)
File "c:\python27\lib\subprocess.py", line 958, in _execute_child
startupinfo)
TypeError: environment can only contain strings
```