Enable contrib init file and RPM's spec file to run on RHEL 6/CentOS 6 a...

It is happy if I can use a gitbucket RPM package on RHEL 6/CentOS 6, and can run a gitbucket process as non root user.

Current gitbucket repository have a minimum feature RPM spec file: contrib/linux/redhat/gitbucket.spec,
init script file: contrib/gitbucket.ini.

The gitbucket.spec file and gitbucket.init file enable the gitbucket process to run as root user.
And the current gitbucket.init script file cannot run on RHEL 6/CentOS 6 by error.
So, I modified the gitbucket.spec file, and created gitbucket.init script file for only RHEL 6/CentOS 6.