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 https://github.com/IBM-Swift/BlueRSA/commit/a35c877669bac43fa6b79fd12668636e6d8dad21 but I do not yet understand what the appropriate fix is, so this test is currently failing.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

:white_check_mark: dannys42
:x: djones6
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[sonarqubecloud[bot]]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities
0 Security Hotspots
2 Code Smells

No Coverage information
0.0% Duplication