Fix `tessel update` on Windows

My test is the following:
```
tessel update -w 1.26
tessel update -b 0.1.10
tessel update
```

This succeeds on:
- [x] Ubuntu 14.04
- [x] Win7 in VirtualBox

It should be tested on:
- [x] Windows 8 @ekolker 
- [x] OS X