fix mkdocs 1.1 compatibility

mkdocs 1.1 removed util.string_types and recommends to use str directly

Fixes: zhaoterryy#12

mkdocs_git_revision_date_plugin/plugin.py

@@ -3,14 +3,13 @@
 
 from mkdocs.config import config_options
 from mkdocs.plugins import BasePlugin
-from mkdocs.utils import string_types
 from jinja2 import Template
 from .util import Util
 
 
 class GitRevisionDatePlugin(BasePlugin):
     config_scheme = (
-        ('enabled_if_env', config_options.Type(string_types)),
+        ('enabled_if_env', config_options.Type(str)),
         ('modify_md', config_options.Type(bool, default=True)),
         ('as_datetime', config_options.Type(bool, default=False)),
     )
@@ -56,4 +55,4 @@ def on_page_markdown(self, markdown, page, config, files):
                 return markdown
         else:
             md_template = Template(markdown)
-            return md_template.render({'git_revision_date': revision_date})
+            return md_template.render({'git_revision_date': revision_date})