pcreate output_dir: base dir name 'pyramid' is forbidden

Solution for issue https://github.com/Pylons/pyramid/issues/1357 where pcreate creates project named pyramid.

New behavior:
```
$ virtualenv pyramid
$ cd pyramid
$ . bin/activate
$ pip install pyramid
$ pcreate -s starter .
You must change output_dir. Base dir name 'pyramid' is forbidden.
```