Bump nbrsessionproxy version

For https://github.com/jupyterhub/mybinder.org-deploy/issues/865

repo2docker/buildpacks/r.py

@@ -214,7 +214,7 @@ class RBuildPack(PythonBuildPack):
                 "${NB_USER}",
                 # Install nbrsessionproxy
                 r"""
-                pip install --no-cache-dir nbrsessionproxy==0.7.0 && \
+                pip install --no-cache-dir nbrsessionproxy==0.8.0 && \
                 jupyter serverextension enable nbrsessionproxy --sys-prefix && \
                 jupyter nbextension install --py nbrsessionproxy --sys-prefix && \
                 jupyter nbextension enable --py nbrsessionproxy --sys-prefix