setup: Use older version of pypandoc for setuptools-markdown

`setuptools-markdown` relies on a deprecated function
`pypandoc.convert`. This patch explicitly requests an older version of
`pypandoc` until a patch has been released for `setuptools-markdown`:
- msabramo/setuptools-markdown#21

In addition, the environment variable `NO_PANDOC` has been introduced
to allow to build and install `docker-inside` without adding
`setuptools-markdown` and `pypandoc` as a build time dependency
(`setup_requires`).

Author: boon-code

setup.py

@@ -21,12 +21,22 @@
     else:
         __version__ = "{0}.{1}".format(__version__, build_id)
 
+setup_extra = dict()
+if os.environ.get("NO_PANDOC", "") == "":  # use pandoc
+    setup_extra['long_description_markdown_filename'] = 'README.md'
+    setup_extra['setup_requires'] = [
+        'setuptools-markdown>=0.4.1',
+        # pypandoc.convert is deprecated but setuptools-markdown still relies on it
+        # -> so use an older version of pypandoc that still provides this until
+        # https://github.com/msabramo/setuptools-markdown/pull/21 is released
+        'pypandoc>=1.2.0,<1.3.0'
+    ]
+
 
 setup(
     name='docker-inside',
     version=__version__,
     description='Run a docker container with you workspace and user',
-    long_description_markdown_filename='README.md',
     url="https://github.com/boon-code/docker-inside",
     license=__license__,
     author=__author__,
@@ -56,5 +66,5 @@
         "docker>=2.7.0",
         "dockerpty>=0.4.1"
     ],
-    setup_requires=['setuptools-markdown'],
+    **setup_extra
 )