Bump OP version in setup.py

I ran into a weird bug where installing the latest OP on a system that already has an installation from a version prior to commit de26da3 results in the following error message:

$ onionperf
Traceback (most recent call last):
  File "/usr/local/bin/onionperf", line 4, in <module>
    __import__('pkg_resources').run_script('OnionPerf==0.2rc0', 'onionperf')
  File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 667, in run_script
    self.require(requires)[0].run_script(script_name, ns)
  File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 1452, in run_script
    raise ResolutionError(
pkg_resources.ResolutionError: Script 'scripts/onionperf' not found in metadata at None

It looks like setuptools is renaming the OnionPerf version from 0.2.pre (as it is currently specified in setup.py) to 0.2rc0 when installing, and they somehow conflict. I've tested this can be solved by bumping the version to 0.3 in setup.py (patch attached).