ci: Update to 4.2.3, official Docker images and enable NIO testing

[djones6]

Description

• Update to official Swift Docker images (speeds up build time)
• Update to Swift 4.2.3
• Re-enable NIO mode testing now that Add %-encoding in ClientRequest.init, remove in HTTPServerRequest.init Kitura-NIO#171 and Update the value of ClientResponse.httpStatusCode Kitura-NIO#172 are merged.

Motivation and Context

Resolves #91

How Has This Been Tested?

Checklist:

• I have submitted a CLA form
• If applicable, I have updated the documentation accordingly.
• If applicable, I have added tests to cover my changes.

[pushkarnk]

@djones6 The only failure in the latest run is with NIO on macOS, which seems to be due to the unavailability of openssl. I think we'd need to add BREW_INSTALL_PACKAGES="libressl" to the corresponding Travis step.

[djones6]

@pushkarnk Thanks - CI is running clean now, so this is ready to review/merge.

[pushkarnk]

LGTM. Just one question. We seem to have removed all 14.04 builds from Travis. Was that intended?

[djones6]

@pushkarnk The removal of 14.04 builds is due to 14.04 shortly going out of service. There will be extended support as a paid offering, but I expect Kitura users will migrate (or have already migrated) to 16.04 or higher. The official Swift Docker images are based on 16.04.

I submitted Kitura/Kitura#1429 to restore one permutation of 14.04 testing for the Kitura repo (this will also cover all Kitura's dependencies), and will retain one similarly in Kitura-Sample (which has a broader set of dependencies including Stencil, Sessions, Credentials, SwiftMetrics - though not CouchDB).

I could be persuaded to reintroduce 14.04 to a broader set of repos if we feel there's real risk of breakage in the future. At the moment I see it as very low risk, confined to places where we directly interact with C libraries (curl, openssl, zlib) that are back-level on 14.04.