Update to Swift 5.1, fix crash on Linux

[djones6]

This PR updates the CI configuration to use the Swift 5.1 release toolchain.

While testing, I noticed an intermittent failure in test_makeKeyPair() that results in an error being thrown on macOS, and a hard crash on Linux (force-unwrapping nil):

macOS:

/Users/travis/build/IBM-Swift/BlueRSA/Tests/CryptorRSATests/CryptorRSATests.swift:770:
  error: -[CryptorRSATests.CryptorRSATests test_makeKeyPair] : failed - test_makeKeyPair 
  failed for bitSize: 1024, with error: Error code: -9999(0x-270F), Couldn't create key 
  reference from key data

Linux:

Test Case 'CryptorRSATests.test_makeKeyPair' started at 2019-09-27 15:15:58.221
Fatal error: Unexpectedly found nil while unwrapping an Optional value

I've included a fix here for the force-unwrap: d2i_PUBKEY_bio and d2i_PrivateKey_bio can fail (returning null) if the BIO does not contain a valid RSA key, but we were not handling that case. The test still fails intermittently, but with an error now:

/home/travis/build/IBM-Swift/BlueRSA/Tests/CryptorRSATests/CryptorRSATests.swift:770:
  error: CryptorRSATests.test_makeKeyPair : failed - test_makeKeyPair failed for bitSize: 
  4096, with error: Error code: -9999(0x-270F), ERROR: Failed to create key from BIO, 
  code: 218570875, reason: header too long

I tracked down the reason for the intermittent failure in #52 - an unsafe assumption that leads us - occasionally - to incorrectly drop the first 26 bytes of the key data. I added a test for this situation in a35c877 but I do not yet understand what the appropriate fix is, so this test is currently failing.

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✅ dannys42
❌ djones6
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Kudos, SonarCloud Quality Gate passed!

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0.0% Duplication

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Quality Gate passed

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2 New issues
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0.0% Coverage on New Code
0.0% Duplication on New Code

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[dannys42]

Thanks @djones6 !