Fix missing argument in file.replace

I was trying to use `file.replace` with `prepend_if_not_found` and it was throwing this exception:

```
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/salt/cli/caller.py", line 130, in call
    ret['return'] = func(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/salt/modules/file.py", line 1165, in replace
    new_file.insert(not_found_content + '\n')
TypeError: insert() takes exactly 2 arguments (1 given)
```

Reproducable with `touch /tmp/b; salt-call file.replace /tmp/b pattern=a repl=b prepend_if_not_found=True`