Fix pom-export of Maven Central repository

Commit a1e26ca6 (in PR #1507) broke the `make-pom` & `pom-advanced` dependency-management tests by replacing one reference to `IBiblioResolver.DEFAULT_M2_ROOT` in `Resolver`, but not the other reference in `MakePom`:

https://travis-ci.org/sbt/sbt/jobs/31939788#L2517-L2519

...the secure url was no longer recognised as the default Maven Repository root, so was erroneously exported.

Apologies for not running the full test suite sooner...

cc @eed3si9n