Use HTTPS for downloading artifacts from Maven Central

Sonatype have enabled HTTPS access for Maven Central:

http://central.sonatype.org/articles/2014/Aug/03/https-support-launching-now/

Note that the Ivy class IBiblioResolver contains the old http url (ie `DEFAULT_M2_ROOT="http://repo1.maven.org/maven2/"`):

http://svn.apache.org/viewvc/ant/ivy/core/trunk/src/java/org/apache/ivy/plugins/resolver/IBiblioResolver.java?revision=1557968&view=markup#l72

An alternative to this PR would be to wait for that artifact to update, but I can't think of any reason to not just include the url directly, as done with the other repository roots.