Fixed issues: SimpleDateFormat 'Y' and lein midje-doc once

To successfully generate docs, I had to overcome two small issues:

#### 1. SimpleDateFormat 'Y'
```shell
$ lein midje-doc
Retrieving lein-midje-doc/lein-midje-doc/0.0.18/lein-midje-doc-0.0.18.pom from clojars
Retrieving lein-midje-doc/lein-midje-doc/0.0.18/lein-midje-doc-0.0.18.jar from clojars
Generating doc/map.html
#<IllegalArgumentException java.lang.IllegalArgumentException: Illegal pattern character 'Y'>
Error Generating doc/map.html
```
^ Fixed by a4abc4e

Maybe the 'Y' only exists for your locale?
> SimpleDateFormat also supports localized date and time pattern strings. In these strings, the pattern letters described above may be replaced with other, locale dependent, pattern letters.

#### 2. lein midje-doc once
```shell
$ lein midje-doc once
java.lang.IllegalArgumentException: No matching clause: once
        at leiningen.midje_doc$midje_doc.doInvoke(midje_doc.clj:60)
```
^ Fixed by a71b504

The support for `lein midje-doc once` is documented here: http://docs.caudate.me/lein-midje-doc/#generating-from-source; however, you may decide to just correct the doc.