Make distro dependency optional

The distro package only supports Linux, and it is only used
here to detect Ubuntu.  Make it an optional dependency in the
code, so that packagers can exclude it when it isnt needed.

Closes ofek#15

userpath/shells.py

@@ -1,6 +1,9 @@
 from os import path, pathsep
 
-import distro
+try:
+    import distro
+except ImportError:
+    distro = None
 
 DEFAULT_SHELLS = ('bash', 'sh')
 
@@ -34,7 +37,7 @@ def config(self, location, front=True):
         configs = {path.join(self.home, '.bashrc'): contents}
 
         # https://github.com/ofek/userpath/issues/3#issuecomment-492491977
-        if distro.id() == 'ubuntu':
+        if distro and distro.id() == 'ubuntu':
             login_config = path.join(self.home, '.profile')
         else:
             # NOTE: If it is decided in future that we want to make a distinction between