Fixes to enforce xgboost==0.90

Issue raised to upgrade xgboost version

CONTRIBUTING.md

@@ -154,6 +154,8 @@ currently using a minimum version of PyCharm 2019.2.4.
 -   Setup Elasticsearch instance (assumes `localhost:9200`), and run
     `python -m eland.tests.setup_tests` to setup test environment -*note
     this modifies Elasticsearch indices*
+-   Install local `eland` module (required to execute notebook tests)
+    `python install setup.py`
 -   Run `pytest --nbval  --doctest-modules` to validate install
 
 ### Documentation

docs/requirements-docs.txt

@@ -6,4 +6,4 @@ git+https://github.com/pandas-dev/pandas-sphinx-theme.git@master
 numpydoc>=0.9.0
 nbsphinx
 scikit-learn
-xgboost
+xgboost==0.90

requirements-dev.txt

@@ -5,4 +5,4 @@ pytest>=5.2.1
 nbval
 numpydoc>=0.9.0
 scikit-learn>=0.22.1
-xgboost>=0.90
+xgboost==0.90