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>

.bazelversion

@@ -0,0 +1 @@
+0.26.1