Update Dockerfile because php:apache updated

[zuphilip]

The php:apache now uses also Debian 7 with PHP 7 and therfore
we need to replace the package php5-dev with php-dev.

See also #85.

[zuphilip]

Hm.. not yet working, see https://hub.docker.com/r/ubma/malibu/builds/bwl5j9ddkttfwwqhczwm2nu/
"
Package 'php-dev' has no installation candidate E: Package 'php-pear' has no installation candidate �[0m Package php-pear is not available, but is referred to by another package. This may mean that the package is missing, has been obsoleted, or is only available from another source Package php-dev is not available, but is referred to by another package. This may mean that the package is missing, has been obsoleted, or is only available from another source
"

[zuphilip]

Okay, the docker image build successfully now without the packages php-dev and php-pear: https://hub.docker.com/r/ubma/malibu/builds/. I have read that

When you install php* packages from Debian, you install a second copy of PHP (or at least some of its modules), and not the version you're intending to use from this image.

Are these packages not anymore needed at all in Debian? CC @stweil

[zuphilip]

@stweil Can you look at this PR and also possible changes for our installation on a Debian system? For more information see https://github.com/docker-library/docs/tree/master/php#e-package-php-xxx-has-no-installation-candidate

[zuphilip]

Merged 🎉 CC @jmechnich