Don't require maven-metadata.xml to be of modelVersion 1.1.0

[aconstantin]

While working with a nexus repository server it may occur that the maven-metadata.xml of a certain versions is not (yet) containing information about all snapshotVersions directly. This may be accomplished by a background task.

<metadata>
  <groupId>my-group</groupId>
  <artifactId>my-group</artifactId>
  <version>1.0.0-SNAPSHOT</version>
  <versioning>
    <snapshot>
      <timestamp>20150626.101100</timestamp>
      <buildNumber>4</buildNumber>
    </snapshot>
    <lastUpdated>20150626101130</lastUpdated>
  </versioning>
</metadata>

If the maven-metadata.xml is enhanced it will contain an additional list of snapshotVersions and a root attribute modelVersion="1.1.0" which will look like the following example

<metadata>
  <groupId>my-group</groupId>
  <artifactId>my-group</artifactId>
  <version>1.0.0-SNAPSHOT</version>
  <versioning>
    <snapshot>
      <timestamp>20150626.101100</timestamp>
      <buildNumber>4</buildNumber>
    </snapshot>
    <lastUpdated>20150626101130</lastUpdated>
    <snapshotVersions>
      <snapshotVersion>
        <classifier>javadoc<classifier>
        <extension>jar<extension>
        <value>1.0.0-20150626.101100.1<value>
        <updated>20150626101130</updated>
      </snapshotVersion>
    </snapshotVersions>
  </versioning>
</metadata>

Currently only a maven-metadata.xml with `modelVersion="1.1.0"`` is supported. This would be good to change.