Failing test on Ubuntu 16.04 (xenial)

[ianpartridge]

Example: https://travis-ci.org/RuntimeTools/SwiftMetrics/jobs/524976223

Test Case 'CoreSwiftMetricsTests.SwiftMetricsMemory' started at 2019-04-26 14:32:43.817
[Fri Apr 26 14:32:43 2019] com.ibm.diagnostics.healthcenter.loader INFO: Swift Application Metrics
/home/travis/build/RuntimeTools/SwiftMetrics/Tests/CoreSwiftMetricsTests/CoreSwiftMetricsTests.swift:136: error: CoreSwiftMetricsTests.SwiftMetricsMemory : Asynchronous wait failed - Exceeded timeout of 10.0 seconds, with unfulfilled expectations: Expect a Memory event
/home/travis/build/RuntimeTools/SwiftMetrics/Tests/CoreSwiftMetricsTests/CoreSwiftMetricsTests.swift:137: error: CoreSwiftMetricsTests.SwiftMetricsMemory : XCTAssertNil failed: "XCTestError(_nsError: The operation could not be completed)" - 
Test Case 'CoreSwiftMetricsTests.SwiftMetricsMemory' failed (10.008 seconds)

For now, I've disabled testing on 16.04 but we need to get it working (and 18.04) as 14.04 is ancient history.