Update pep8 to 1.5.7

This is just to keep us on the upgrade treadmill. ([Changelog](https://pypi.python.org/pypi/pep8#changelog))
[Here's a list of PEP8 error codes.](http://pep8.readthedocs.org/en/latest/intro.html#error-codes).

* E501: line too long. Note that we can set the `max-line-length` in the `setup.cfg` file, so if we want pep8 to alert when we go beyond 120 characters instead of the default 79, we can set `max-line-length` to `120` and remove `E501` from the `ignore` list.
* E265: block comment should start with `# `. There are lots of places where we have comments that look like this:
```
############### ACTIONS ################
```
Which I think is a perfectly reasonable sort of comment, but violates this guideline because it doesn't start with a hash followed by a space. As a result, I just set it to ignore.
* W602: deprecated form of raising exception. We do this in two places: [capa/inputtypes.py](https://github.com/edx/edx-platform/blob/master/common/lib/capa/capa/inputtypes.py#L235) and [xmodule/xml_module.py](https://github.com/edx/edx-platform/blob/master/common/lib/xmodule/xmodule/xml_module.py#L181). In both cases, we do it to modify the exception message but preserve the traceback: @nedbat [has a blogpost about this technique](http://nedbatchelder.com/blog/200711/rethrowing_exceptions_in_python.html). It's a bit strange, but we have a perfectly valid reason for doing it this way, and in fact, at least one unit test broke when I tried changing it. So I left it as-is and ignored this warning instead.