Add 'activate' note for conda versions from 4.4

* `conda activate` has been the preferred commands since version 4.4
(released December 2017)
* `source activate` does not work in all environments

CONTRIBUTING.md

@@ -43,11 +43,14 @@ Activate the 2.7 environment and install nose and lifelines
  - source activate thinkstats2-py27
  - pip install nose lifelines
 
-Activate the 3.6 environment and install nose
+Activate the 3.6 environment and install nose and lifelines
 
  - source activate thinkstats2-py36
  - pip install nose lifelines
 
+*For conda versions after 4.4 use `conda activate` instead of `source activate`*
+
+
 Extras that are helpful --
 
  - runipy allows you to run IPython Notebooks on the command line which