Use string interpolation, to avoid segmentation fault.

The segmentation fault was ocurring when precompiling assets, with ruby2.0.0.
Fix .pkgr.yml so that discourse can be packaged with https://pkgr.io.
Add link to the main Discourse Plugin page for the PLUGINS doc.

Sorry for all of the crazy commits. I got my branches mixed up, I
should be good now.

This adds a link to the main discourse plugin page
Merge pull request #2186 from pkgr/pkgr

Automatically generate a debian package
Merge pull request #2189 from willkara/doc-edits

Add link to the main Discourse Plugin page for the PLUGINS doc.