create-pom includes indexistent version of maven-clean-plugin

### Steps to Reproduce

1. grails create-app test
2. cd test
3. grails create-pom org.test

### Expected Behaviour
The plugin maven-clean-plugin must be added as it is but with an existent version. For example:

```

org.apache.maven.plugins
maven-clean-plugin
2.4.1



plugins

**/*

false

```

### Actual Behaviour

The plugin maven-clean-plugin is included but with an inexistent version.

```

org.apache.maven.plugins
maven-clean-plugin
2.4.0



plugins

**/*

false

```

### Environment Information

- **Operating System**: Ubuntu 16.04.2
- **Grails Version:** 2.5.6
- **JDK Version:** 1.6.0_33

### Suggested solution
Replace the version in the file `src/grails/templates/maven/project.pom` to an existent version (for example 2.4.1)

Inspect src/grails/templates/maven/project.pom and reproduce the grails create-app, create-pom flow described in the issue. Update the maven-clean-plugin version from 2.4.0 to an existing version, then verify that the generated POM contains the corrected version and preserves its configuration.

Written by the indexing model from the issue text.