Added check curl (required by {redhat,centos} builds)

If curl package is missing on build server, {redhat,centos} builds
fail with a not explicate message from rpm.

rpm: RPM should not be used directly install RPM packages, use Alien
instead!

Closes: #65
Merge pull request #66 from enovance/bug/65/sbadia

Added check curl (required by {redhat,centos} builds)