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
Update misc tests and doc to reflect HTTPS access for Maven Central
scalariform project/Docs.scala
Adds sbt.repository.secure (default: true) #1494
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 #1507 from sbt/wip/optional-https-maven-central

"Use HTTPS for downloading artifacts from Maven Central" + disable flag