version: Fail gracefully if git describe cannot get version.

When running the `./tools/cache-zulip-git-version` script on Travis, the script
fails because Travis gets a shallow clone of the repository, and not a full
clone. This commit changes the script to fail gracefully, if we are unable to
get the version information using `git describe`.

When the command fails, it still writes an empty `zulip-git-version` and that
has not been changed to keep creation of the release tarball simple, and
avoiding a check for whether the file has any content. The code that sets
`ZULIP_VERSION` checks whether the contents of the `zulip-git-version` file are
empty, before setting `ZULIP_VERSION`. So, the version should never be set to an
empty string.

tools/cache-zulip-git-version

@@ -2,4 +2,4 @@
 set -e
 
 cd "$(dirname "$0")/.."
-git describe --tags > zulip-git-version
+git describe --tags > zulip-git-version || true