ngx_mruby v2.2.4

• update test target to 1.19.1 #468 by @sawanoboly
• update test target to 1.19.2 #470 by sawanoboly
• Fix Nginx::Headers_in[] returns wrong value. Fixed #471. #472 by @yyamano
• Fix segmentation fault when set nil to Nginx::Headers_{in,out} #475 by yyamano
• Ignore build_config.rb.lock generated by mruby build system. #476 by yyamano
• fix typo on doc/install/README.md #477 by @d6rkaiz
• fix document link on ngx_mruby gh_pages. #478 by d6rkaiz
• update test target to 1.19.3 #479 by sawanoboly
• update test target to 1.19.4 #480 by sawanoboly
• update test target to 1.19.5 #482 by sawanoboly
• update test target to 1.19.6, and openssl 1.1.1i #485 by sawanoboly
• transformed travis test to github actions #486 by sawanoboly
• update test target to 1.19.8 #487 by sawanoboly
• update test target to 1.19.9 #488 by sawanoboly
• update test target to 1.19.10 #489 by sawanoboly
• Update test target to 1.21.0(mainline) and add 1.20.1(stable) #490 by sawanoboly
• update test target to 1.21.1 #491 by sawanoboly
• update test target to 1.21.3 #492 by sawanoboly