(PUP-2878) file conflict upgrading from 2.7.26 to 3.6.2

Currently, upgrades from 2.7.26 to 3.6.2 break because of a file
conflict. It seems that as of Puppet 3.3.0, the manpages changed from
being owned by the puppet package, to being owned by the puppet-common
package. This change was initially done to allow upgrades from Debian
packages to ours. However, changing which package owns a given file
makes upgrades difficult. This commit adds in conflicts to allow smooth
upgrades from puppet packages prior to the manpage change to the current
version.
This line isn't strictly necessary, but it seemed like it was cleaner to add it in