Add configs for files marked as `text/javascript`

By default the configurations from this repository ensure that
`.js` files are served with the `application/javascript` media
type, compressed, and with far future expires headers. However,
not all users¹ use all the configurations provided by this
project, or, in some cases, the headers are overwritten from the
level "beneath" Apache (e.g.: from `php`).

For most of these cases we can't do anything about it.
Nonetheless, there is one, quite common case (the media type is
set to `text/javascript`) where we can help.

Since the `text/javascript` media type is used² quite often, this
commit makes Apache serve anything labeled with this media type,
compressed and with far future expires headers.

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¹ h5bp/server-configs-apache#27
  h5bp/server-configs-apache#41

² Even the Google CND serves `.js` files with the `text/javascript`
  media type (e.g.: http://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js)