USE_BAZEL_VERSION with absolute path results in "invalid version unknown"

When doing this:

```
% USE_BAZEL_VERSION=$PWD/tmp/bazel/versions/1.0.0-darwin/bin/bazel bazelisk --migrate build //...
2019/10/16 11:41:45 could not get the list of incompatible flags: invalid version unknown
```

Is this because my bazel binary doesn't have the version in the binary name itself?

Start by reproducing the documented command with USE_BAZEL_VERSION set to the absolute Bazel binary path, then trace Bazelisk's version detection and incompatible-flags lookup. Done means the command no longer reports "invalid version unknown" when given that path, with behavior verified by an appropriate regression test.

Written by the indexing model from the issue text.