Remove `mrblib_dir` and `objs_dir` configuration from gems.

Always use `mrblib` and 'src` for directory names.

lib/mruby/gem.rb

@@ -17,7 +17,6 @@ module MRuby
       attr_accessor :name, :dir, :build
       alias mruby build
       attr_accessor :build_config_initializer
-      attr_accessor :mrblib_dir, :objs_dir
 
       attr_accessor :version
       attr_accessor :description, :summary
@@ -45,8 +44,6 @@ module MRuby
         @name = name
         @initializer = block
         @version = "0.0.0"
-        @mrblib_dir = "mrblib"
-        @objs_dir = "src"
         @dependencies = []
         @conflicts = []
         MRuby::Gem.current = self
@@ -61,8 +58,8 @@ module MRuby
         end
         @linker = LinkerConfig.new([], [], [], [], [])
 
-        @rbfiles = Dir.glob("#{@dir}/#{@mrblib_dir}/**/*.rb").sort
-        @objs = Dir.glob("#{@dir}/#{@objs_dir}/*.{c,cpp,cxx,cc,m,asm,s,S}").map do |f|
+        @rbfiles = Dir.glob("#{@dir}/mrblib/**/*.rb").sort
+        @objs = Dir.glob("#{@dir}/src/*.{c,cpp,cxx,cc,m,asm,s,S}").map do |f|
           objfile(f.relative_path_from(@dir).to_s.pathmap("#{build_dir}/%X"))
         end