Do not crash on using version check with development version of bazel. (

#145)

workspace.bzl

@@ -2,7 +2,10 @@ load("@bazel_skylib//lib:versions.bzl", "versions")
 
 def _store_bazel_version(repository_ctx):
     bazel_version = versions.get();
-    if versions.is_at_most("0.17.0", bazel_version):
+    if len(bazel_version) == 0:
+      print("You're using development build of Bazel, " +
+            "make sure it's at least version 0.17.1")
+    elif versions.is_at_most("0.17.0", bazel_version):
         fail("Bazel %s is too old to use with rules_rust, please use at least Bazel 0.17.1, preferably newer." % bazel_version)
     repository_ctx.file("BUILD", "exports_files(['def.bzl'])")
     repository_ctx.file("def.bzl", "BAZEL_VERSION='" + bazel_version + "'")