handle uninstalled pkg in __version__ assignment

Author: majuscule

README.rst

@@ -37,8 +37,12 @@ To use setuptools_scm just modify your project's setup.py file like this:
 
    .. code:: python
 
-      import pkg_resources
-      __version__ = pkg_resources.get_distribution(__name__).version
+      from pkg_resources import get_distribution, DistributionNotFound
+      try:
+          __version__ = get_distribution(__name__).version
+      except DistributionNotFound:
+         # package is not installed
+         pass
 
 
 Programmatic usage