Correct passenger curl dependencies for RHEL-famly v6+

As of RHEL 6, `curl-devel` no longer exists and is replaced with `libcurl-devel`. `yum` will still resolve the dependency, but must then do so by effectively doing a 'provides' scan of all the configured repositories for the host, potentially consuming a large amount of system memory depending on yum repositories configured. For example, on low-memory (<1GB) Oracle Enterprise 6 hosts, yum will actually run the system out of memory while trying to resolve `curl-devel`.