speficy versions of build.yml and env.yml

build/build.yml

@@ -8,7 +8,6 @@ dependencies:
 - pandas=0.23.2
 - notebook=5.0.0
 - pip:
-  - pyopenssl>= 17.3.0
   - nbsphinx==0.2.14
   - recommonmark==0.4.0
   - https://github.com/mli/notedown/tarball/master

build/env.yml

 name: d2l-zh-build
 dependencies:
-- python==3.6
-- jupyter
-- sphinx
-- matplotlib
-- pandas
-- notebook
+- python=3.6
+- jupyter=1.0.0
+- sphinx=1.6.3
+- matplotlib=2.2.2
+- pandas=0.23.2
+- notebook=5.0.0
 - pip:
-  - nbsphinx
-  - recommonmark
+  - nbsphinx==0.2.14
+  - recommonmark==0.4.0
   - https://github.com/mli/notedown/tarball/master
   - mxnet-cu92==1.2.1
   - gluonbook==0.8.5
-  - jieba
+  - jieba==0.39
   - awscli