CentOS support

This change updates the RepositoryIndex functionality to substitute
the 'centos#' as a value for platform.  It also brings the platform
resolution strategy inline with the way the java-buildpack-dependecy-
builder does it to ensure that the values will match.

[#58119316]
Merge 58119316-centos-support to master

[#58119316]
Centos6 support documentation

This change adds the missed update to the documentation that points to
the centos6 repository.

[#58119316]
Merge 58119316-centos-support to master

[#58119316]
Merge branch 'master' of github.com:cloudfoundry/java-buildpack