Update logstash.spec

Fixed the following warnings:
[root@localhost rpmbuild]# rpmbuild -bb SPECS/logstash.spec
varning: bogus date in %changelog: Fri May 6 2013 sjir@basefarm.se 1.1.10-3
varning: bogus date in %changelog: Sat Feb  1 2013 Richard Pijnenburg <richard@i                                    spavailability.com> - 1.1.9-1
varning: bogus date in %changelog: Fri May 6 2013 sjir@basefarm.se 1.1.10-3
varning: bogus date in %changelog: Sat Feb  1 2013 Richard Pijnenburg <richard@i                                    spavailability.com> - 1.1.9-1