Add description of `RUBY_CFLAGS` to README.
manually reverted using mirror site. and use https protocols
use http, because http://ftp.ruby-lang.org is deliveried by fastly
rename hostname, ftp.ruby-lang.org is used by ftp protocol too.
Merge pull request #409 from hsbt/revert-using-mirror

Manually reverted using mirror site. and use http protocols
Merge pull request #408 from alloy/patch-1

Add description of `RUBY_CFLAGS` to README.
ruby-build 20130907
Fix typo in the README file
Merge pull request #412 from ebouchut/typo-fixes

Fix a typo in the README file.