Remove square-brace syntax

Dependencies specified inside of square-brackets are rather troublesome in both ZSH and BASH (both of which try to glob their contents).

* [ ] Remove support for `[...]`-style arguments to the plugin.
* [ ] Update README.md to suggest `lein try dep-name`-style, then `lein try dep-name version-num` as a follow-on.
* [ ] Cut a 0.4.0 release. (@rkneufeld only, unless @xsc wants push and @rkneufeld docs the release process.)

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Explanation of possible BASH behavior:

>  nDuff:	 rkneufeld: so, [123] will be delivered to the program as '[123]', *unless* the current directory contains a file named 1, a file named 2, or a file named 3, or the "nullglob" shell option is available.