Small improvement: add z3c.relationfield, plone.app.contenttypes in install_requires/setup.py

Using a really minimal Plone 4 installation (https://github.com/collective/minimalplone4/) we get these errors when adding collective.exportimport to eggs:

```
collective.exportimport-1.0-py2.7.egg/collective/exportimport/configure.zcml", line 13.2-19.8
ImportError: No module named relationfield.interfaces

collective.exportimport-1.0-py2.7.egg/collective/exportimport/configure.zcml", line 57.2-58.52
ImportError: No module named contenttypes.interfaces
```

Funny that this wasn't caught on tests.

Needed other pins as well, but is out of the scope of this project (will put them here for the sake of documentation):

```
# Python 2/Plone 4 compatibility.
plone.restapi = 6.13.8
PyJWT = 1.7.1
# https://github.com/Julian/jsonschema/issues/453
# Getting distribution for 'pyrsistent>=0.14.0'
# ValueError: need more than 0 values to unpack
jsonschema = 2.6.0
```

Inspect setup.py and its install_requires entries, then review configure.zcml where the missing relationfield and contenttypes imports fail. Reproduce the minimal Plone 4 installation error if possible; done means those imports resolve with the declared dependencies and the project tests pass.

Written by the indexing model from the issue text.