Use bazelisk to switch between used bazel version

Use bazelisk to switch between used bazel version

Bazelisk is utility to switch to the right Bazel version, that we
used to have with Buck build tool: [1].

Bazelisk will download the right Bazel version only once and will use
it in subsequent calls:

  $ bazelisk build :release
  2019/06/06 16:22:15 Downloading \
  https://releases.bazel.build/0.26.1/release/bazel-0.26.1-linux-x86_64...

Bazelisk is storing the binaries in user's cache directory: [2], e.g.
on Linux OS:

  $ ls -1 ~/.cache/bazelisk/bin
  bazel-0.26.1-linux-x86_64

* [1] https://github.com/bazelbuild/bazelisk
* [2] https://golang.org/pkg/os/#UserCacheDir

Change-Id: Ia9180fb75f8cc17a0a0232622cf33a13bfad6b60
Signed-off-by: David Ostrovsky <david@ostrovsky.org>