Workaround certain words being reserved in Vim

The following is illegal in Vim:

```viml
syntax keyword clojureFunc foo contains baz
```

It causes "E395: contains argument not accepted here". It seems that other plugins have worked around the issue using `contain[s]`. So that is what I've done here.

Note that this is an issue for anyone using Midje in tests (which is how I hit it in a project):

```clojure
(:require [midje.sweet :refer :all])
```