Add wagtail 2.8 support

Author: tleguijt

.gitignore

@@ -13,5 +13,5 @@
 /htmlcov/
 /node_modules/
 /wagtailstreamforms.egg-info/
-
+/build
 file*.mp4

setup.py

@@ -17,7 +17,7 @@
 
 
 install_requires = [
-    'wagtail>=2,<2.8'
+    'wagtail>=2,<2.9'
 ]
 
 documentation_extras = [
@@ -60,7 +60,6 @@
         'Framework :: Wagtail',
         'Framework :: Wagtail :: 2',
         'Programming Language :: Python :: 3',
-        'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',