Re-enable the dependencies of the spec file

RHEL 6.5 and CentOS 6.5 can satify the required dependencies by adding
the OpenStack related RPM packages. So I'm assuming we can just
re-enable the commented "Requires" section in the spec file.

This resolves MN1550.

Signed-off-by: Taku Fukushima <tfukushima@midokura.com>
Merge pull request #151 from tfukushima/spec-dependencies-master

Re-enable the dependencies of the spec file