Add Bazel version check

Bazel version check was implemented on master, and stable branches, that
were already migrated to Bazel, missied that feature. We need the
version check, to add bazel caching feature, that was only added in very
recent Bazel versions. This is done in follow-up change.

Change-Id: I901caa6f9c037be29d377dd9f1c451d57ecc252f

WORKSPACE

@@ -4,6 +4,13 @@ load("//tools/bzl:maven_jar.bzl", "maven_jar", "GERRIT", "MAVEN_LOCAL")
 load("//lib/codemirror:cm.bzl", "CM_VERSION", "DIFF_MATCH_PATCH_VERSION")
 load("//plugins:external_plugin_deps.bzl", "external_plugin_deps")
 
+http_archive(
+    name = "bazel_skylib",
+    sha256 = "bbccf674aa441c266df9894182d80de104cabd19be98be002f6d478aaa31574d",
+    strip_prefix = "bazel-skylib-2169ae1c374aab4a09aa90e65efe1a3aad4e279b",
+    urls = ["https://github.com/bazelbuild/bazel-skylib/archive/2169ae1c374aab4a09aa90e65efe1a3aad4e279b.tar.gz"],
+)
+
 http_archive(
     name = "io_bazel_rules_closure",
     sha256 = "6691c58a2cd30a86776dd9bb34898b041e37136f2dc7e24cadaeaf599c95c657",
@@ -20,6 +27,10 @@ http_file(
     url = "https://raw.githubusercontent.com/google/closure-compiler/775609aad61e14aef289ebec4bfc09ad88877f9e/contrib/externs/polymer-1.0.js",
 )
 
+load("@bazel_skylib//:lib.bzl", "versions")
+
+versions.check(minimum_bazel_version = "0.7.0")
+
 load("@io_bazel_rules_closure//closure:defs.bzl", "closure_repositories")
 
 # Prevent redundant loading of dependencies.