MAINT: revert back to pip installs using quantecon-book-theme=0.1.7 (#132)

* revert back to pip quantecon-book-theme=0.1.5

* specify 0.1.6

* use pip style for version ==

* fetch latest from pip for quantecon-book-theme

Author: mmcky

environment.yml

@@ -8,7 +8,7 @@ dependencies:
   - pip:
     - jupyter-book
     - sphinx-multitoc-numbering
-    - git+https://github.com/quantecon/quantecon-book-theme
+    - quantecon-book-theme
     - sphinx-tojupyter
     - sphinxext-rediraffe
     - sphinx-exercise