remove cruft in config_files work & simplify code

docs/source/config_files.rst

@@ -95,16 +95,6 @@ version of Ubuntu.
 We use ``apt.txt``, for example, to install LaTeX in our
 `example apt.txt for LaTeX <https://github.com/binder-examples/latex/blob/master/apt.txt>`_.
 
-``setup.py``
-~~~~~~~~~~~~
-
-To install your repository like a Python package, you may include a
-``setup.py`` file. repo2docker installs ``setup.py`` files by running
-``pip install -e .``.
-
-While one can specify dependencies in ``setup.py``,
-repo2docker **requires configuration files such as** ``environment.yml`` or
-``requirements.txt`` to install dependencies during the build process.
 
 ``DESCRIPTION``
 ~~~~~~~~~~~~~~~

repo2docker/buildpacks/r.py

@@ -69,11 +69,13 @@ class RBuildPack(PythonBuildPack):
         """
         # If no date is found, then self.checkpoint_date will be False
         # Otherwise, it'll be a date object, which will evaluate to True
-        if bool(self.checkpoint_date):
+        if self.checkpoint_date:
             return True
         description_R = 'DESCRIPTION'
         if not os.path.exists('binder') and os.path.exists(description_R):
             if not self.checkpoint_date:
+                # no R snapshot date set through runtime.txt
+                # set the R runtime to the latest date that is guaranteed to be on MRAN across timezones
                 self._checkpoint_date = datetime.date.today() - datetime.timedelta(days=2)
                 self._runtime = "r-{}".format(str(self._checkpoint_date))
             return True