Unsupported WARC version: 1.1

[example file](https://kiska.b-cdn.net/omESX/example.warc.gz)
```python
f = warc.open("example.warc.gz")
for record in f:
print record['WARC-Target-URI'], record['Content-Length']
```
### Expected Behaviour
Prints records with URI + Content Length

### Observed Behaviour:
Traceback (most recent call last):
File "", line 1, in
File "/home/kiska/.local/lib/python2.7/site-packages/warc/warc.py", line 390, in __iter__
record = self.read_record()
File "/home/kiska/.local/lib/python2.7/site-packages/warc/warc.py", line 373, in read_record
header = self.read_header(fileobj)
File "/home/kiska/.local/lib/python2.7/site-packages/warc/warc.py", line 334, in read_header
raise IOError("Unsupported WARC version: %s" % version)
IOError: Unsupported WARC version: 1.1