Added chainmap for python 2.7.

README.rst

@@ -4,7 +4,7 @@ invoker
 A wrapper around `invoke <http://www.pyinvoke.org/>`_ to help
 facilitate sharing tasks and specifying multiple environments
 to run tasks against for instance running tasks against staging
-and production environments.
+and production environments. It supports Python 2.7 and 3.4+.
 
 Configuration
 -------------

invoker/loader.py

@@ -2,8 +2,11 @@
 
 import six
 from invoke import Collection, Task
-from collections import ChainMap
 
+try:
+    from collections import ChainMap
+except ImportError:
+    from chainmap import ChainMap
 
 _app_spec_defaults = {
     'namespace': None,

requirements.in

@@ -1 +1,2 @@
 invoke
+chainmap

requirements.txt

@@ -6,6 +6,7 @@
 #
 atomicwrites==1.2.1       # via pytest
 attrs==18.2.0             # via hypothesis, pytest
+chainmap==1.0.2
 coverage==4.5.1
 hypothesis==3.76.0
 invoke==1.2.0