Fix pom-export of Maven Central repository

Commit a1e26ca6 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.
Merge pull request #1511 from rtyley/fix-makepom-with-https-maven-central-root

Fix pom-export of Maven Central repository