Explicitly specify version of jupyter-server-proxy needed

We made a release!

environment.yml

@@ -3,5 +3,4 @@ channels:
 dependencies:
   - websockify
   - pip:
-    - https://github.com/jupyterhub/jupyter-server-proxy/archive/0e67e1afd0bab1342443f13bd147a2f8c682e9e0.zip
     - .

setup.py

@@ -10,7 +10,7 @@ setup(
             'desktop = jupyter_desktop:setup_desktop',
         ]
     },
-    install_requires=['jupyter-server-proxy'],
+    install_requires=['jupyter-server-proxy>=1.2.0'],
     include_package_data=True,
     zip_safe=False
 )