chore(composer): pull version from composer.json

This would allow us to use grunt-bump to increase version numbers
programmatically to reduce release-process overhead.

Also opens to the door to install Elgg with composer:

```
composer create-project elgg/elgg ./path/to/elgg
```

Refs #6288
chore(release): conventional-changelog support

Refs #6783
Merge pull request #6807 from ewinslow/automate-b6783

conventional-changelog support