bazelisk migrate from really old version

[meisterT]

I have a project which builds with bazel 0.15 but doesn't build with bazel at HEAD (I know I should have updated in between) so I have ~15 Bazel versions to test my project with. Now, it would be really great if you could tell bazelisk which version is the last known Bazel version with which my project did successfully build and bazelisk should try with each new released Bazel version since then and compile a report. If the build fails with Bazel X and fails with Bazel X+1, then bazelisk should additionally repeat the build with the --migrate option with Bazel version X.