Having trouble running the "Getting started" section (even the tests do not run)

Hi,

I am trying to run the KnowBert code but keep getting stuck with the following error when trying to run the tests or the "How to embed sentences or sentence pairs programmatically" snippet:

```
tests/evaluation/test_wic_reader.py:3: in
from kb.include_all import WicDatasetReader
kb/include_all.py:2: in
from kb.kg_embedding import KGTupleReader, KGTupleModel
kb/kg_embedding.py:8: in
from allennlp.data import DatasetReader, Token, Vocabulary
/opt/conda/envs/knowbert/lib/python3.6/site-packages/allennlp/data/__init__.py:1: in
from allennlp.data.dataset_readers.dataset_reader import DatasetReader
/opt/conda/envs/knowbert/lib/python3.6/site-packages/allennlp/data/dataset_readers/__init__.py:10: in
from allennlp.data.dataset_readers.ccgbank import CcgBankDatasetReader
/opt/conda/envs/knowbert/lib/python3.6/site-packages/allennlp/data/dataset_readers/ccgbank.py:9: in
from allennlp.data.dataset_readers.dataset_reader import DatasetReader
/opt/conda/envs/knowbert/lib/python3.6/site-packages/allennlp/data/dataset_readers/dataset_reader.py:4: in
from allennlp.data.instance import Instance
/opt/conda/envs/knowbert/lib/python3.6/site-packages/allennlp/data/instance.py:4: in
from allennlp.data.fields import TextField, ListField, SpanField
/opt/conda/envs/knowbert/lib/python3.6/site-packages/allennlp/data/fields/__init__.py:7: in
from allennlp.data.fields.array_field import ArrayField
/opt/conda/envs/knowbert/lib/python3.6/site-packages/allennlp/data/fields/array_field.py:10: in
class ArrayField(Field[numpy.ndarray]):
/opt/conda/envs/knowbert/lib/python3.6/site-packages/allennlp/data/fields/array_field.py:45: in ArrayField
@overrides
/opt/conda/envs/knowbert/lib/python3.6/site-packages/overrides/overrides.py:88: in overrides
return _overrides(method, check_signature, check_at_runtime)
/opt/conda/envs/knowbert/lib/python3.6/site-packages/overrides/overrides.py:114: in _overrides
_validate_method(method, super_class, check_signature)
/opt/conda/envs/knowbert/lib/python3.6/site-packages/overrides/overrides.py:135: in _validate_method
ensure_signature_is_compatible(super_method, method, is_static)
/opt/conda/envs/knowbert/lib/python3.6/site-packages/overrides/signature.py:93: in ensure_signature_is_compatible
ensure_return_type_compatibility(super_type_hints, sub_type_hints, method_name)
/opt/conda/envs/knowbert/lib/python3.6/site-packages/overrides/signature.py:288: in ensure_return_type_compatibility
f"{method_name}: return type `{sub_return}` is not a `{super_return}`."
E TypeError: ArrayField.empty_field: return type `None` is not a `allennlp.data.fields.field.Field`.
```

## Steps to reproduce

1. Enter a clean anaconda docker container `docker run -it continuumio/anaconda3`
2. `apt-get update && apt install -y g++ make`
3. `git clone https://github.com/allenai/kb.git`
4. `conda create -n knowbert python=3.6.7`
5. `source activate knowbert`
6. `pip install torch==1.2.0`
7. `pip install -r requirements.txt`
8. `python -c "import nltk; nltk.download('wordnet')"`
9. `python -m spacy download en_core_web_sm`
10. `pip install --editable .`
11. `pytest -v tests`