Bug#347258: More odd messages in en_GB.po

[Andrew Deason <adeason@tjhsst.edu>]

There are a few other bugs in en_GB.po, I think. Namely, the "Abort."
message is missing a period, and "The following packages have been kept
back:" is missing that final colon.

$ nl en_GB.po | grep -B 2 "kept back\""
   574  #: cmdline/apt-get.cc:423
   575  msgid "The following packages have been kept back:"
   576  msgstr "The following packages have been kept back"
$ nl en_GB.po | grep -B 2 Abort\"
   712  #: cmdline/apt-get.cc:874 cmdline/apt-get.cc:893
   713  msgid "Abort."
   714  msgstr "Abort"
$