Whats new, mrjob?

Updates `whats-new.rst`. However, I haven't proofed it because I'm at a loss as to how to build the docs.

Here's the error I get:

```sh
~/git/mrjob (whats_new_mrjob) $ python setup.py build_sphinx
running build_sphinx
Running Sphinx v1.1.3
loading pickled environment... done
building [html]: all source files
updating environment: 0 added, 0 changed, 0 removed
looking for now-outdated files... none found
preparing documents... Traceback (most recent call last):
  File "setup.py", line 74, in <module>
    **setuptools_kwargs
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/distutils/core.py", line 152, in setup
    dist.run_commands()
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/distutils/dist.py", line 953, in run_commands
    self.run_command(cmd)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/distutils/dist.py", line 972, in run_command
    cmd_obj.run()
  File "/Users/davidmarin/mrjob-test/lib/python2.7/site-packages/sphinx/setup_command.py", line 143, in run
    app.build(force_all=self.all_files)
  File "/Users/davidmarin/mrjob-test/lib/python2.7/site-packages/sphinx/application.py", line 200, in build
    self.builder.build_all()
  File "/Users/davidmarin/mrjob-test/lib/python2.7/site-packages/sphinx/builders/__init__.py", line 159, in build_all
    self.build(None, summary='all source files', method='all')
  File "/Users/davidmarin/mrjob-test/lib/python2.7/site-packages/sphinx/builders/__init__.py", line 252, in build
    self.write(docnames, list(updated_docnames), method)
  File "/Users/davidmarin/mrjob-test/lib/python2.7/site-packages/sphinx/builders/__init__.py", line 283, in write
    self.prepare_writing(docnames)
  File "/Users/davidmarin/mrjob-test/lib/python2.7/site-packages/sphinx/builders/html.py", line 338, in prepare_writing
    self.theme.get_options(self.theme_options).iteritems())
  File "/Users/davidmarin/mrjob-test/lib/python2.7/site-packages/sphinx/theming.py", line 131, in get_options
    raise ThemeError('unsupported theme option %r given' % option)
sphinx.errors.ThemeError: unsupported theme option 'html_show_sourcelink' given
```