Fix fix `file` reference

The `file()` builtin was removed in Python 3. When `TemplateStream.dump()` is called, the following error is raised:

    File "/Users/natan/.virtualenvs/robowolves/lib/python3.3/site-packages/jinja2/environment.py", line 1065, in dump
      fp = file(fp, 'w')
    NameError: global name 'file' is not defined

Replacing it with `open()` appears to work -  I ran the unit tests on both Python 3.3 and Python 2.7 with no failures, but if someone could double-check them for me, that would be great.

However, this is not a complete fix - once the code in this pull request is applied, the following error occurs:

      File "/Users/natan/.virtualenvs/plaster/src/jinja2/build/lib/jinja2/environment.py", line 1075, in dump
        fp.writelines(iterable)
    TypeError: must be str, not bytes

I'm not sure on how I should go about resolving this particular error.