enstaller 4.7.5 crash

Unfortunately, rev 4.7.5 also crashes on my windows machine:

```
$ python bootstrap.py
Downloading https://s3.amazonaws.com/enstaller-assets/enstaller/enstaller-4.7.5-py2.7.egg
enstaller-4.7.5-py2.7.egg [removing egg]
enstaller-4.7.5-py2.7.egg [installing egg]
exit()

C:\Windows\System32>enpkg --version
C:\Python27\lib\site-packages\enstaller\vendor\keyring\core.py:166: UserWarning:
Module argparse was already imported from C:\Python27\lib\argparse.pyc, but c:\
python27\lib\site-packages is being added to sys.path
pkg_resources = __import__('pkg_resources')
enstaller version: 4.7.5

C:\Windows\System32>enpkg --whats-new
C:\Python27\lib\site-packages\enstaller\vendor\keyring\core.py:166: UserWarning: Module argparse was already imported from C:\Python27\lib\argparse.py, but c:\python27\lib\site-packages is being added to sys.path pkg_resources = __import__('pkg_resources')
/eggs/win-64/index.json [Fetching index]
1.05 MB [......................................................]
enstaller: Error: enstaller crashed (uncaught exception : KeyError('python',)).
Please report this on enstaller issue tracker:
http://github.com/enthought/enstaller/issues
You can get a full traceback by setting the ENSTALLER_DEBUG environment variable

C:\Windows\System32> set ENSTALLER_DEBUG=TRUE
C:\Windows\System32>enpkg --whats-new
C:\Python27\lib\site-packages\enstaller\vendor\keyring\core.py:166: UserWarning: Module argparse was already imported from C:\Python27\lib\argparse.py, but c:\python27\lib\site-packages is being added to sys.path
pkg_resources = __import__('pkg_resources')
/eggs/win-64/index.json [Fetching index]
1.05 MB [......................................................]
Traceback (most recent call last):
File "C:\Python27\Scripts\enpkg-script.py", line 10, in
sys.exit(main_noexc())
File "C:\Python27\lib\site-packages\enstaller\main.py", line 873, in main_noexc
main(argv)
File "C:\Python27\lib\site-packages\enstaller\main.py", line 755, in main
enpkg = Enpkg(repository, downloader, prefixes, progress_bar_context)
File "C:\Python27\lib\site-packages\enstaller\enpkg.py", line 289, in __init__
self._installed_repository = Repository._from_prefixes(self.prefixes)
File "C:\Python27\lib\site-packages\enstaller\repository.py", line 295, in _fr
om_prefixes
repository._populate_from_prefixes(prefixes)
File "C:\Python27\lib\site-packages\enstaller\repository.py", line 281, in _po
pulate_from_prefixes
InstalledPackageMetadata.from_installed_meta_dict(info)
File "C:\Python27\lib\site-packages\enstaller\repository.py", line 192, in fro
m_installed_meta_dict
json_dict["python"], json_dict["ctime"],
KeyError: 'python'
```