Merge pull request #35 from VinceMacBuche/bug_4081/2.5_2.8

Fixes #4081: Upgrade instruction from 2.5 to 2.8
Fixes #4124: User manual doesn't mention Debian wheezy support
Fixes #4125: User manual doesn't mention RHEL/CentOS 3 support
Merge pull request #38 from jooooooon/bug_4124/user_manual_doesn_t_mention_debian_wheezy_support

Fixes #4124: User manual doesn't mention Debian wheezy support
Merge pull request #39 from jooooooon/bug_4125/user_manual_doesn_t_mention_rhel_centos_3_support

Fixes #4125: User manual doesn't mention RHEL/CentOS 3 support
Merge branch 'branches/rudder/2.4' into branches/rudder/2.6
Merge branch 'branches/rudder/2.6' into branches/rudder/2.7
Merge branch 'branches/rudder/2.7' into branches/rudder/2.8
Fixes #4122: Update version of Ubuntu maintained