Update easy-thumbnails requirement from <2.5 to <2.7

Updates the requirements on [easy-thumbnails](https://github.com/SmileyChris/easy-thumbnails) to permit the latest version.
- [Release notes](https://github.com/SmileyChris/easy-thumbnails/releases)
- [Changelog](https://github.com/SmileyChris/easy-thumbnails/blob/master/CHANGES.rst)
- [Commits](SmileyChris/easy-thumbnails@2.4.1...2.6.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements-docs.txt

@@ -1,4 +1,4 @@
 -r requirements-test.txt
 Django<2.0
 html5lib<0.99999999
-easy-thumbnails<2.5
+easy-thumbnails<2.7