Fixing an issue with inline sed on Darwin

I noticed an issue with Darwin `sed`, inline with no backups. Current behaviour is:

```Python
sed('filename', key, value, backup='')
```
Gives me:
```Bash
$ sed -i -E -e 's/key/value/g' "$(echo filename)"
...
$ ls
filename
filename-E
```
This seems to be because `sed` on Darwin behaves slightly differently when parsing the `-i` option. This patch seems to fix. Fabric seems to use Darwin `sed` even when `gnu-sed` is installed as default with homebrew:
```bash
run: sed --version
out: sed (GNU sed) 4.2.2
```