Previously successful Travis build now fails when build is restarted

[matteobilardi]

The last commit https://github.com/matteobilardi/flint/commit/2c1c8d8179031ecc39908a6771e3d9353c330df2
was successfully building and passing tests on Travis before I restarted the build for that commit at a later date. In particular, the Lite dependency now fails to import POSIX.

Since the codebase has not changed since the last time the build was run (and all the Swift modules that the codebase depends on are fixed at versions that are known to be working), the failure can likely be attributed to a change in the build environment (e.g. a different version of packages being downloaded by the docker instance).