text: Title: Syntax error running jupyter-repo2docker
  Body: I ran into a syntax error after installing `jupyter-repo2docker` in my conda environment.
  
  I get the following when I try to do anything with jupyter-repo2docker.
  
  ```sh
  admins-mbp-3:~ jacquelineburos$ jupyter-repo2docker --version
  Traceback (most recent call last):
    File "/Users/jacquelineburos/anaconda3/bin/jupyter-repo2docker", line 7, in <module>
      from repo2docker.__main__ import main
    File "/Users/jacquelineburos/anaconda3/lib/python3.5/site-packages/repo2docker/__main__.py", line 1, in <module>
      from .app import Repo2Docker
    File "/Users/jacquelineburos/anaconda3/lib/python3.5/site-packages/repo2docker/app.py", line 592
      self.log.info(f'Successfully pushed {self.output_image_spec}', extra=dict(phase='pushing'))
                                                                  ^
  SyntaxError: invalid syntax
  ```
  
  Looks like your python_requires should be updated to >=3.6 since you are using f-strings.
  
  Here is my system setup:
  
  ```py
  Current conda install:
  
                 platform : osx-64
            conda version : 4.3.22
         conda is private : False
        conda-env version : 4.3.22
      conda-build version : 0+unknown
           python version : 3.5.2.final.0
         requests version : 2.14.2
         root environment : /Users/jacquelineburos/anaconda3  (writable)
      default environment : /Users/jacquelineburos/anaconda3
         envs directories : /Users/jacquelineburos/anaconda3/envs
                            /Users/jacquelineburos/.conda/envs
            package cache : /Users/jacquelineburos/anaconda3/pkgs
                            /Users/jacquelineburos/.conda/pkgs
             channel URLs : https://repo.continuum.io/pkgs/free/osx-64
                            https://repo.continuum.io/pkgs/free/noarch
                            https://repo.continuum.io/pkgs/r/osx-64
                            https://repo.continuum.io/pkgs/r/noarch
                            https://repo.continuum.io/pkgs/pro/osx-64
                            https://repo.continuum.io/pkgs/pro/noarch
              config file : /Users/jacquelineburos/.condarc
               netrc file : None
             offline mode : False
               user-agent : conda/4.3.22 requests/2.14.2 CPython/3.5.2 Darwin/17.7.0 OSX/10.13.6
                  UID:GID : 502:20
  ```