explicitly put pip in the base env

instead of depending on conda's automatic pip dependency for Python,
which is sensitive to configuation

repo2docker/buildpacks/conda/environment.py-2.7.yml

@@ -2,5 +2,6 @@ channels:
 - conda-forge
 dependencies:
 - python=2.7.*
+- pip
 - ipykernel==4.8.2
 - wcwidth==0.1.9

repo2docker/buildpacks/conda/environment.yml

@@ -2,6 +2,7 @@ channels:
   - conda-forge
 dependencies:
   - python=3.7
+  - pip
   - ipywidgets==7.6.3
   - jupyter-offlinenotebook==0.2.1
   - jupyter-resource-usage==0.5.1