Refactor Dockerfile, switching the base from `buildpack-deps` over to `debian` directly

This results in a dramatic reduction of image size:

```console
$ docker images php
REPOSITORY          TAG                 IMAGE ID            CREATED              VIRTUAL SIZE
php                 5.4                 6b03f428509e        3 hours ago          354.3 MB
php                 5.4-apache          2bd126f2c469        2 minutes ago        394.8 MB
php                 5.5                 19a5113df69a        9 minutes ago        356.4 MB
php                 5.5-apache          6c892721307f        2 minutes ago        396.9 MB
php                 5.6                 69aead156fe7        10 minutes ago       356.8 MB
php                 5.6-apache          2d0bae628aaf        About an hour ago    397.3 MB
```

vs the old:

```console
$ docker images php
REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
php                 5.4                 e760b9bd2c6c        2 days ago          736.9 MB
php                 5.4-apache          62c23e8f1704        2 days ago          811.8 MB
php                 5.5                 156d2a323bc0        2 days ago          739.8 MB
php                 5.5-apache          3014d0f81b29        2 days ago          814.7 MB
php                 5.6                 dab86418397c        2 days ago          740.2 MB
php                 5.6-apache          b3c4a0633954        2 days ago          815.1 MB
```

Fixes #33