Fix module redefinition errors related to CommonCrypto and zlib

[pixelspark]

When building Starscream in a project together with Kitura-WebSocket (or other IBM packages that use crypto), the compiler complains about there being two 'CommonCrypto' modules. See 'swift package show-dependencies' output:

├── Kitura-WebSocket<https://github.com/IBM-Swift/Kitura-WebSocket@0.8.2>
│   ├── Kitura-net<https://github.com/IBM-Swift/Kitura-net.git@1.7.12>
....
│   │   └── SSLService<https://github.com/IBM-Swift/BlueSSLService.git@0.12.44>
│   │       └── Socket<https://github.com/IBM-Swift/BlueSocket.git@0.12.55>
│   └── Cryptor<https://github.com/IBM-Swift/BlueCryptor.git@0.8.14>
│       └── CommonCrypto<https://github.com/IBM-Swift/CommonCrypto.git@0.1.4>
└── Starscream<https://github.com/daltoniam/Starscream.git@2.1.0>
    ├── zlib<https://github.com/daltoniam/zlib-spm.git@1.0.0>
    └── CommonCrypto<https://github.com/daltoniam/common-crypto-spm.git@1.0.0>

This PR makes Starscream use the IBM module (which essentially is the same kind of wrapper around CommonCrypto as is daltoniam/common-crypto-spm), which resolves the issue (and leaves you with one less repository to maintain 🙂).

[pixelspark]

Also, the zlib module is defined by the system as of XCode 9... which leads to 'module redefinition' errors. Should change the name of the zlib module to something else (e.g. "CZLib") and use that instead (or remove the module altogether, but that only works >=XCode 9). See PR daltoniam/zlib-spm#1

[daltoniam]

Thanks for the PR (I merged the other one on zlib repo). I had to do some updates to the pod spec too, so I just pulled this changes in manually. Thanks again.

[pixelspark]

Cool thanks!