Update Serpent hash to 4.2

[clackary]

Pull Request Description

The xfail for https://bugs.swift.org/browse/SR-8250 is still valid, and is bumped to 4.2 along with the BuildSwiftPackage configuration.

• pass ./project_precommit_check script run

./project_precommit_check Serpent --earliest-compatible-swift-version 4.2
** CHECK **
--- Validating Serpent Swift version 4.2 compatibility ---
--- Project configured to be compatible with Swift 4.2 ---
--- Checking Serpent platform compatibility with Darwin ---
--- Platform compatibility check succeeded ---
--- Locating swiftc executable ---
$ xcrun -f swiftc
--- Checking installed Swift version ---
$ /Applications/Xcodes/Xcode_10_Beta_6.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_6.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swiftc --projects projects.json --include-repos 'path == "Serpent"' --include-versions 'version == "4.2"' --include-actions 'action.startswith("Build")'
PASS: Serpent, 4.2, 33ce18, Swift Package
========================================
Action Summary:
     Passed: 1
     Failed: 0
    XFailed: 0
    UPassed: 0
      Total: 1
========================================
Repository Summary:
      Total: 1
========================================
Result: PASS
========================================
--- Serpent checked successfully against Swift 4.2 ---

[clackary]

@swift-ci test