Update montage-wrapper to use astropy-helpers

`montage-wrapper` should be updated from `package-template` (to use `astropy-helpers`).

Currently I see this error (but I think it's harmless):
```
$ git clone git@github.com:astropy/montage-wrapper.git
$ cd montage-wrapper
$ python setup.py build
Freezing version number to montage_wrapper/version.py
running build
running build_py
creating build
creating build/lib.macosx-10.9-x86_64-2.7
creating build/lib.macosx-10.9-x86_64-2.7/montage_wrapper
copying montage_wrapper/__init__.py -> build/lib.macosx-10.9-x86_64-2.7/montage_wrapper
copying montage_wrapper/commands.py -> build/lib.macosx-10.9-x86_64-2.7/montage_wrapper
copying montage_wrapper/commands_extra.py -> build/lib.macosx-10.9-x86_64-2.7/montage_wrapper
copying montage_wrapper/conftest.py -> build/lib.macosx-10.9-x86_64-2.7/montage_wrapper
copying montage_wrapper/mpi.py -> build/lib.macosx-10.9-x86_64-2.7/montage_wrapper
copying montage_wrapper/status.py -> build/lib.macosx-10.9-x86_64-2.7/montage_wrapper
copying montage_wrapper/version.py -> build/lib.macosx-10.9-x86_64-2.7/montage_wrapper
copying montage_wrapper/wrappers.py -> build/lib.macosx-10.9-x86_64-2.7/montage_wrapper
creating build/lib.macosx-10.9-x86_64-2.7/montage_wrapper/tests
copying montage_wrapper/tests/__init__.py -> build/lib.macosx-10.9-x86_64-2.7/montage_wrapper/tests
copying montage_wrapper/tests/test_wrappers.py -> build/lib.macosx-10.9-x86_64-2.7/montage_wrapper/tests
generating default montage_wrapper.cfg file in montage_wrapper/montage_wrapper.cfg
Generation of default configuration item failed! Stdout and stderr are shown below.
Stdout:

Stderr:
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/deil/code/astropy/astropy/config/configuration.py", line 1050, in generate_all_config_items
    _save_config(package.__name__, filename=filename)
  File "/Users/deil/code/astropy/astropy/config/configuration.py", line 977, in _save_config
    sec.write(outfile=f)
  File "/Users/deil/code/astropy/astropy/extern/configobj_py2/configobj.py", line 2109, in write
    outfile.write(output)
TypeError: must be unicode, not str
```