added SHELL declaration so that make is able to find mapcat

Prior to adding the SHELL declaration make was aborting because it couldn't find mapcat in my npm bin:

```
% make
cd pkg && make all
make[1]: mapcat: No such file or directory
make[1]: *** [annotator.js] Error 1
make: *** [all] Error 2
```

It seems that unless you set PATH explicitly it falls back to using the user's shell which doesn't have the altered PATH. At least that was my hasty reading of [the doc](https://www.gnu.org/software/make/manual/html_node/Choosing-the-Shell.html). I'm guessing that others who have run this happened to have mapcat installed globally so they never noticed this.

I also added the document plugin to the list of plugins package, and switched back to spec reporter for tests since I like to see which tests are actually running.