'clean' seems to be ignored

I'm using the following:

```
middleman: {
            options: {
            },
            server: {
			},
            build: {
                options: {
                    command: "build",
                    clean: false // This does not work
                }
            }
        }
```
with:
`grunt.registerTask('build', ['middleman:build','sass','concat']);`

Unfortunately, it continues to 'clean'. This commit fixes this behaviour and restores the Middleman default.