Better ignore syntaxerror stack traces in wheels

This already got stripped just fine for non-wheel installs, but for
wheels, it only stripped the lined where `SyntaxError` was present
leaving the rest of the stacktrace.

This has fixed a bug left behind with #1984, but inside that PR, this logic was explicitly done. So I have no idea if I'm breaking something by doing this.

With my patch:

```
$ pip install https://pypi.python.org/packages/2.7/g/gunicorn/gunicorn-19.1.1-py2.py3-none-any.whl\#md5\=905b2a090447c82bd66e62ee29b2287c
DEPRECATION: --download-cache has been deprecated and will be removed in the future. Pip now automatically uses and configures its cache.
Downloading/unpacking gunicorn==19.1.1 from https://pypi.python.org/packages/2.7/g/gunicorn/gunicorn-19.1.1-py2.py3-none-any.whl#md5=905b2a090447c82bd66e62ee29b2287c
  Using cached gunicorn-19.1.1-py2.py3-none-any.whl
Installing collected packages: gunicorn
Compiling /Users/matt/.virtualenvs/pip/build/gunicorn/gunicorn/workers/_gaiohttp.py


Successfully installed gunicorn
Cleaning up...
```

Without:

```
$ pip install https://pypi.python.org/packages/2.7/g/gunicorn/gunicorn-19.1.1-py2.py3-none-any.whl\#md5\=905b2a090447c82bd66e62ee29b2287c
DEPRECATION: --download-cache has been deprecated and will be removed in the future. Pip now automatically uses and configures its cache.
Downloading/unpacking gunicorn==19.1.1 from https://pypi.python.org/packages/2.7/g/gunicorn/gunicorn-19.1.1-py2.py3-none-any.whl#md5=905b2a090447c82bd66e62ee29b2287c
  Using cached gunicorn-19.1.1-py2.py3-none-any.whl
Installing collected packages: gunicorn
Compiling /Users/matt/.virtualenvs/pip/build/gunicorn/gunicorn/workers/_gaiohttp.py ...
  File "/Users/matt/.virtualenvs/pip/build/gunicorn/gunicorn/workers/_gaiohttp.py", line 64
    yield from self.wsgi.close()
             ^

Successfully installed gunicorn
Cleaning up...
```