fix swizzle for newer pyobjc.

the `swizzle` function fails for me on OSX 10.10 with:
```
  File "/Users/rygwdn/.virtualenvs/aioTV/lib/python2.7/site-packages/pytest_osxnotify/__init__.py", line 22, in pytest_sessionstart
    notify('py.test', 'Running tests...')
  File "/Users/rygwdn/.virtualenvs/aioTV/lib/python2.7/site-packages/pytest_osxnotify/__init__.py", line 59, in notify
    swizzled_bundleIdentifier)
  File "/Users/rygwdn/.virtualenvs/aioTV/lib/python2.7/site-packages/pytest_osxnotify/__init__.py", line 44, in swizzle
    old_IMP = cls.instanceMethodForSelector_(SEL)
AttributeError: No selector bundleIdentifier
```
using `getattr` instead of `instanceMethodForSelector_` seems to fix the issue for me.