[ruby-cvs:71175] normal:r64084 (trunk): use https:// instead of git:// when possible

[normal at ruby-lang.org]

normal	2018-07-28 12:34:30 +0900 (Sat, 28 Jul 2018)

  New Revision: 64084

  https://svn.ruby-lang.org/cgi-bin/viewvc.cgi?view=revision&revision=64084

  Log:
    use https:// instead of git:// when possible
    
    Avoid MitM when downloading from insecure networks.

  Modified files:
    trunk/README.md
    trunk/common.mk
    trunk/doc/contributing.rdoc
    trunk/tool/make-snapshot