make 20th stable release

[tsahara]

• Add build_mrbtest after config block is evaluated mruby/mruby#2940 - not applicable
• Added range.h.md and edited re.h.md and version.h.md mruby/mruby#2941 - documentation
• Revert 7b5f8b0 except removing trailing spaces mruby/mruby#2942 - not applicable
• Allow rbfiles in mrblib and test to have subdirs mruby/mruby#2943 - feature change
• Releases mruby/mruby#2944 - question
• Keys of hash are modifyable, leads to unexpected behavior mruby/mruby#2945 - change request
• Add gem test dependencies mruby/mruby#2946 - feature change
• freeze the hash keys (fixes #2945) mruby/mruby#2947 - spec change
• Custom install directory mruby/mruby#2948 - change request
• Customize installation directory using INSTALL_DIR environment variable mruby/mruby#2949 - spec change
• mirb should take filename and arguments. mruby/mruby#2950 - change request
• Don't crash if NULL is passed to mrb_close mruby/mruby#2951 - feature change
• mruby@74696ff - feature change
• mruby@3a462fe - feature change
• fix block variable in Hash#fetch mruby/mruby#2952 - not applicable
• support String#[pos, len]= val mruby/mruby#2953 - spec change
• get ruby backtrace from c mruby/mruby#2954 - not a bug
• GC not reclaiming objects mruby/mruby#2955 - not a bug
• Problem: Hash#fetch doesn't raise KeyError when a key cannot be found mruby/mruby#2956 - feature change
• Replace 1 with TURE macro mruby/mruby#2957 - style change
• Fix indents mruby/mruby#2958 - style change
• C API documentation mruby/mruby#2959 - documentation
• Refator Hash#fetch test mruby/mruby#2960 - refactoring
• support Regexp literal option: //n and //u mruby/mruby#2961 - spec change
• LinkerConfig is a struct with five params mruby/mruby#2962 - not a bug
• String#inspect(UTF-8String) does not work correctly mruby/mruby#2963 - not applicable
• fix String#inspect with MRB_UTF8_STRING mruby/mruby#2964 - not applicable
• fix signature of mrb_class_new_instance() mruby/mruby#2965 - not applicable
• Small Markdown updates to README.md mruby/mruby#2966 - documentation
• String#chop(UTF-8String) removes last byte, not character mruby/mruby#2967 - not applicable
• Array#index is missing its block overload mruby/mruby#2968 - feature change
• on Ubuntu 14.04, yields "undefined reference" mruby/mruby#2969 - not a bug
• use Enumerable#find_index as Array#index mruby/mruby#2970 - feature change
• chop with utf-8. fix #2967 mruby/mruby#2971 - not applicable
• way to specify/check flags via build_config.rb mruby/mruby#2972 - change request
• fix tests on windows. mruby/mruby#2973 - not applicable

[tsahara]

There are no changes applicable to stable_1_0.