'...' is not a postfix unary operator on Travis CI - xcode

Question

I've been slowly updating an old project and suddenly started getting this error from Travis CI...
2018-05-20 13:32:21.393 xcodebuild[1348:5913] Error Domain=IDETestOperationsObserverErrorDomain Code=4 "Test operation was canceled. If you believe this error represents a bug, please attach the log file at /Users/travis/Library/Developer/Xcode/DerivedData/CF_Apps-eihscxvvnylkkecahehxquvcotoo/Logs/Test/2B963612-D4B6-4CE6-BD58-FC9C49FFAE11/Session-CF Apps Tests-2018-05-20_133203-xth6Px.log" UserInfo={NSLocalizedDescription=Test operation was canceled. If you believe this error represents a bug, please attach the log file at /Users/travis/Library/Developer/Xcode/DerivedData/CF_Apps-eihscxvvnylkkecahehxquvcotoo/Logs/Test/2B963612-D4B6-4CE6-BD58-FC9C49FFAE11/Session-CF Apps Tests-2018-05-20_133203-xth6Px.log}
2018-05-20 13:32:21.394 xcodebuild[1348:7053] Connection peer refused channel request for "dtxproxy:XCTestManager_IDEInterface:XCTestManager_DaemonConnectionInterface"; channel canceled <DTXChannel: 0x7fb70286f820>
Testing failed:
'...' is not a postfix unary operator
'count' is unavailable: there is no universally good answer, see the documentation comment for discussion
Cannot convert value of type 'Int' to expected argument type 'IntMax' (aka 'Int64')
Cannot convert value of type 'Int8' to expected argument type 'IntMax' (aka 'Int64')
Cannot convert value of type 'Int16' to expected argument type 'IntMax' (aka 'Int64')
Cannot convert value of type 'Int32' to expected argument type 'IntMax' (aka 'Int64')
Cannot convert value of type 'UInt' to expected argument type 'UIntMax' (aka 'UInt64')
Cannot convert value of type 'UInt8' to expected argument type 'UIntMax' (aka 'UInt64')
Cannot convert value of type 'UInt16' to expected argument type 'UIntMax' (aka 'UInt64')
Cannot convert value of type 'UInt32' to expected argument type 'UIntMax' (aka 'UInt64')
** TEST FAILED **
The following build commands failed:
CompileSwiftSources normal x86_64 com.apple.xcode.tools.swift.compiler
CompileSwift normal x86_64 /Users/travis/build/osis/cf-apps-ios/Pods/ObjectMapper/Sources/HexColorTransform.swift
CompileSwift normal x86_64 /Users/travis/build/osis/cf-apps-ios/Pods/ObjectMapper/Sources/IntegerOperators.swift
(3 failures)
The command "xcodebuild -workspace CF\ Apps.xcworkspace -scheme CF\ Apps -sdk iphonesimulator -destination 'platform=iOS Simulator,name=iPhone SE' build-for-testing test" exited with 65.
Done. Your build exited with 1.
/Users/travis/.travis/job_stages: line 166: shell_session_update: command not found
I haven't changed any code and this runs properly locally.

Answer 1

While updating many things I overlooked the Xcode bump diff between my local environment and Travis CI.
Changing...
osx_image: xcode8.3
to...
osx_image: xcode9.3
in my .travis.yml fixed this for me.