Update InspIRCd formula from 2.0.17 to 2.0.18.

The dependency on libgcrypt has been removed when building with GnuTLS 2.12+ (inspircd/inspircd@a49fd4cba6a44334eb5a39c6fbe8f6cebd55d0c7) so the associated formula line has been removed.

I have also switched it to use the OpenSSL formula instead of the system OpenSSL as `brew audit` instructed.