Use latest RC version of Marshmallow

tox.ini

@@ -5,23 +5,23 @@ envlist=py{35,36,37,py3}-marshmallow{2,3}, cython-marshmallow{2,3}
 deps=
     -rrequirements/build_common.txt
     marshmallow2: marshmallow <3.0
-    marshmallow3: marshmallow==3.0.0rc6
+    marshmallow3: marshmallow >=3.0.0rc5
 
 whitelist_externals=flake8
 commands=py.test --cov-report html --cov hug -n auto tests
 
 [testenv:py37-black]
 deps=
     -rrequirements/build_style_tools.txt
-    marshmallow==3.0.0rc6
+    marshmallow >=3.0.0rc5
 
 whitelist_externals=flake8
 commands=black --check --verbose -l 100 hug
 
 [testenv:py37-vulture]
 deps=
     -rrequirements/build_style_tools.txt
-    marshmallow==3.0.0rc6
+    marshmallow >=3.0.0rc5
 
 whitelist_externals=flake8
 commands=vulture hug --min-confidence 100 --ignore-names req_succeeded
@@ -30,23 +30,23 @@ commands=vulture hug --min-confidence 100 --ignore-names req_succeeded
 [testenv:py37-flake8]
 deps=
     -rrequirements/build_style_tools.txt
-    marshmallow==3.0.0rc6
+    marshmallow >=3.0.0rc5
 
 whitelist_externals=flake8
 commands=flake8 hug
 
 [testenv:py37-bandit]
 deps=
     -rrequirements/build_style_tools.txt
-    marshmallow==3.0.0rc6
+    marshmallow >=3.0.0rc5
 
 whitelist_externals=flake8
 commands=bandit -r hug/ -ll
 
 [testenv:py37-isort]
 deps=
     -rrequirements/build_style_tools.txt
-    marshmallow==3.0.0rc6
+    marshmallow >=3.0.0rc5
 
 whitelist_externals=flake8
 commands=isort -c --diff --recursive hug