Bump rake-compiler-dock to pick up Ruby 2.6, drop older ones
Bump rake-compiler-dock to pick up Ruby 2.6, drop older ones

With this version bump, the precompiled mysql2 gem for Windows supports
Ruby 2.2, 2.3, 2.4, 2.5, 2.6. Windows Ruby 2.0 and 2.1 are dropped.

sodabrew/mysql2,brianmario/mysql2,jeremy/mysql2,brianmario/mysql2,brianmario/mysql2,sodabrew/mysql2,jeremy/mysql2,sodabrew/mysql2,jeremy/mysql2