Check for gevent presence in concert script

bin/concert

@@ -12,6 +12,7 @@ import tempfile
 import traceback
 import zipfile
 import concert
+import concert.async
 import concert.config
 import concert.session.management as cs
 from concert.helpers import Command
@@ -52,7 +53,7 @@ def get_ipython_shell(config=None):
         from IPython.terminal.embed import InteractiveShellEmbed
         shell = InteractiveShellEmbed(config=config, banner1='')
 
-    if concert.config.ENABLE_GEVENT:
+    if concert.config.ENABLE_GEVENT and concert.async.HAVE_GEVENT:
         if cmp_versions(version, '2.0') < 0:
             from IPython.lib.inputhook import inputhook_manager
             from concert.ext.gevent import inputhook_gevent