Bump to 4.2 in .swift-version file

The .swift-version file is used by swiftenv to determine what version of Swift is used when you build the project. It does not affect what version is used when the driver is included a dependency. It is overridden by an environment variable SWIFT_VERSION when testing on Travis in order to allow testing multiple Swift versions there.

Therefore, modifying the file will only have an effect on driver developers who use swiftenv. The motivation for using 4.2 by default is that it's a lot faster than 4.0. Running make clean; time make on 4.0 takes about 24s, whereas on 4.2 it only takes about 16s.