Updates ObjectMapper hash to build against 4.2

[clackary]

Pull Request Description

This new hash passes project_precommit_check when built against Xcode 10 Beta 3's Swift 4.2 compiler.

./project_precommit_check ObjectMapper --earliest-compatible-swift-version 4.2
** CHECK **
--- Validating ObjectMapper Swift version 4.2 compatibility ---
--- Project configured to be compatible with Swift 4.2 ---
--- Checking ObjectMapper platform compatibility with Darwin ---
--- Platform compatibility check succeeded ---
--- Locating swiftc executable ---
$ xcrun -f swiftc
--- Checking installed Swift version ---
$ /Applications/Xcodes/Xcode-10-beta-3.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swiftc --version
--- Version check succeeded ---
--- Executing build actions ---
$ ./runner.py --swift-branch swift-4.2-branch --swiftc /Applications/Xcodes/Xcode-10-beta-3.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swiftc --projects projects.json --include-repos 'path == "ObjectMapper"' --include-versions 'version == "4.2"' --include-actions 'action.startswith("Build")'
PASS: ObjectMapper, 4.2, ed1caa, ObjectMapper-iOS, generic/platform=iOS
PASS: ObjectMapper, 4.2, ed1caa, ObjectMapper-Mac, generic/platform=macOS
PASS: ObjectMapper, 4.2, ed1caa, ObjectMapper-tvOS, generic/platform=tvOS
PASS: ObjectMapper, 4.2, ed1caa, ObjectMapper-watchOS, generic/platform=watchOS
========================================
Action Summary:
     Passed: 4
     Failed: 0
    XFailed: 0
    UPassed: 0
      Total: 4
========================================
Repository Summary:
      Total: 1
========================================
Result: PASS
========================================
--- ObjectMapper checked successfully against Swift 4.2 ---

It also removes the associated xfail which no longer applies: https://bugs.swift.org/browse/SR-6690
rdar://40863157

[clackary]

@swift-ci test