add-apt-repository moved to package software-properties-common in 14.04

add-apt-repository Ubuntu package:
 - 'python-software-properties' in 12.04 (precise)
 - 'software-properties-common' in 14.04 (trusty)

http://packages.ubuntu.com/precise/all/python-software-properties/filelist
http://packages.ubuntu.com/trusty/all/software-properties-common/filelist