Include PyInvoke

Author: Kinnaird McQuade

requirements-dev.txt

@@ -2,6 +2,7 @@ astroid==2.4.2
 autoflake==1.3.1
 autopep8==1.5.4
 coverage==5.2.1
+invoke==1.4.1
 isort==4.3.21
 lazy-object-proxy==1.4.3
 mccabe==0.6.1

setup.py

@@ -39,7 +39,7 @@ def get_description():
     test_suite='tests/unit',
     tests_require=TESTS_REQUIRE,
     extras_require={
-        'dev': TESTS_REQUIRE + ['autoflake', 'autopep8', 'pylint'],
+        'dev': TESTS_REQUIRE + ['autoflake', 'autopep8', 'pylint', 'invoke'],
         'es1': ['elasticsearch==1.9.0', 'elasticsearch_dsl==0.0.11'],
         'es6': ['elasticsearch==6.1.1', 'elasticsearch_dsl==6.1.0']
     },