Correct dependency installation section

``` clojure
:dependencies [abengoa/clj-stripe "1.0.4"]
```

is syntactically valid Clojure but Leiningen requires `:dependencies` to be a vector of vectors.
So simply tell the user what the artifact/version pair is, the rest can be trivially figured out even by beginners.
Merge pull request #6 from michaelklishin/patch-1

Correct dependency installation section