Encode bytes from popen

* fixes crash on python3 (Archlinux)

Popen returns bytes encoded in the default system encoding (at least on Archlinux python 3.4.0):
```
Traceback (most recent call last):
  File "/usr/bin/virtualenv", line 3, in <module>
    virtualenv.main()
  File "/usr/lib/python3.4/site-packages/virtualenv.py", line 824, in main
    symlink=options.symlink)
  File "/usr/lib/python3.4/site-packages/virtualenv.py", line 980, in create_environment
    home_dir, lib_dir, inc_dir, bin_dir = path_locations(home_dir)
  File "/usr/lib/python3.4/site-packages/virtualenv.py", line 1047, in path_locations
    inc_dir = join(home_dir, 'include', stdout.strip(), py_version + abiflags)
  File "/usr/lib/python3.4/posixpath.py", line 92, in join
    "components.") from None
TypeError: Can't mix strings and bytes in path components.
```

Quite urgent IMHO.
cheers