Use reference tags instead of YAML anchors
This should allow us to be able to redefine `before_script` in
project-specific CI configs. An example is when templates make use of
the .thumbnail() method: the build fails unless imagemagick is
installed.
Thus we can use this in the project's CI config:
```
pages:
before_script:
- !reference [.apt-template, before_script]
- apt-get install imagemagick -y
```