Changelog updated with LDAP and StatsD plugins

Merge pull request Kong#1019 from Mashape/chore/serf

Improving the performance of Serf

chore(rockspec) bump to 0.6.1

Removing print message

version bump

Adding user

Version bump

release: 0.5.4

[hotfix/galileo] path fix

fix(cli) re-enable search in $PATH for nginx executable

Since IO.file_exists was being called and of course returned false when
called with just `nginx`, the command was never executed, and no chance
was given to the shell to try to find the executable in the $PATH.

Fix Kong#610