Update jinja2 to 2.8.0.

Changelog:

* Added target parameter to urlize function.
* Added support for followsymlinks to the file system loader.
* The truncate filter now counts the length.
* Added equalto filter that helps with select filters.
* Changed cache keys to use absolute file names if available instead of load
  names.
* Fixed loop length calculation for some iterators.
* Changed how Jinja2 enforces strings to be native strings in Python 2 to work
  when people break their default encoding.
* Added make_logging_undefined() which returns an undefined object that logs
  failures into a logger.
* If unmarshalling of cached data fails the template will be reloaded now.
* Implemented a block set tag.
* Default cache size was incrased to 400 from a low 50.
* Fixed is number test to accept long integers in all Python versions.
* Changed is number to accept Decimal as a number.
* Added a check for default arguments followed by non-default arguments. This
  change makes {% macro m(x, y=1, z) %}...{% endmacro %} a syntax error. The
  previous behavior for this code was broken anyway (resulting in the default
  value being applied to y).
* Add ability to use custom subclasses of jinja2.compiler.CodeGenerator and
  jinja2.runtime.Context by adding two new attributes to the environment
  (code_generator_class and context_class) (pull request #404).
* added support for context/environment/evalctx decorator functions on the
  finalize callback of the environment.
* escape query strings for urlencode properly. Previously slashes were not
  escaped in that place.
* Add ‘base’ parameter to ‘int’ filter.

requirements.txt

@@ -1,4 +1,4 @@
-Jinja2==2.7.3
+Jinja2==2.8.0
 MarkupSafe==0.23
 Pygments==2.0.2
 pyPEG2==2.15.1