python projects might fail to snap if the package python-dev is not installed

Without libpython-dev, the py2-project example fails to snap with the following error:

 src/pykeccak.c:20:20: fatal error: Python.h: Dosiero aŭ dosierujo ne ekzistas

To reproduce:

# after bug #1501977 has been fixed.
sudo apt-get remove libpython2.7-dev
bzr branch lp:snapcraft
cd snapcraft/examples/py2-project
./../../bin/snapcraft assemble

I'm not sure how to fix this, because the py3-project works without the -dev libs. So I suppose they are not always required and it might be unnecessary to put it in the build-packages of plugins/python.yaml. But maybe it doesn't hurt to be extra careful and add it in there.