dev: make default rake task include rubocop

and remove clobber

Author: flavorjones

Diff for: Rakefile

@@ -5,3 +5,5 @@
 #
 require "bundler"
 SQLITE3_SPEC = Bundler.load_gemspec("sqlite3.gemspec")
+
+task default: [:rubocop, :compile, :test]

Diff for: rakelib/native.rake

@@ -112,8 +112,6 @@ task "set-version-to-timestamp" do
   puts "NOTE: wrote version as \"#{fake_version}\""
 end
 
-task default: [:clobber, :compile, :test]
-
 CLEAN.add("{ext,lib}/**/*.{o,so}", "pkg")
 CLOBBER.add("ports/*").exclude(%r{ports/archives$})