post-install script fails on wheezy with python2

After building this package on wheezy with python2, I found that the post-install script exits 1:
```
Setting up graphite-api (1.0.1-1417544560) ...
dpkg: error processing graphite-api (--configure):
 subprocess installed post-installation script returned error exit status 1
Errors were encountered while processing:
 graphite-api
E: Sub-process /usr/bin/dpkg returned an error code (1)
```
This was when compiling the bytecode:
```
root@host:/var/lib/dpkg/info# bash -x ./graphite-api.postinst
+ set -e
+ python -m compileall /usr/share/python/graphite/lib
root@host:/var/lib/dpkg/info# echo $?
1
```
This is due to a Python3 file inside the Raven project, which is only imported when the interpreter is Python3:
```
root@host:/var/lib/dpkg/info# python -m compileall /usr/share/python/graphite/lib
[...]
Compiling /usr/share/python/graphite/lib/python2.7/site-packages/raven/transport/aiohttp.py ...
SyntaxError: ('invalid syntax', ('/usr/share/python/graphite/lib/python2.7/site-packages/raven/transport/aiohttp.py', 30, 37, '    def __init__(self, parsed_url, *, verify_ssl=True, resolve=True,\n'))
[...]
```
Background is here: https://github.com/getsentry/raven-python/issues/496

I'm not sure of the best way around this, so I've added an ```|| true``` to the bytecode compile when running under python2, as the rest of the files still compile successfully.