support: don't use PPAs on newer Ubuntu releases

This patch adds a new variable NEEDS_PPAS that controls if we should try
to enable PPAs before we try to install packages. It's not perfect (not
granular) but it's good enough for this case and is rather simple to
implement.

In addition, mark 15.04 as supported.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>