Various fixes for Ruby 2.0.0 compatibility

Various fixes for Ruby 2.0.0 compatibility

  - switch Gemfile source from :gemcutter to 'https://rubygems.org'
    (silences security warnings)
  - bump version number prior to 4.0.0 release
  - drop unnecessary `encoding` magic comments from source files
  - drop support for versions of Ruby < 2.0.0
  - make sure string_from_str() returns a string that is marked as UTF-8
    encoded; otherwise a string marked as "ASCII-8BIT" will be returned
    and string comparisons will fail; this was noticed due to the
    breakage of the "redlink" specs which were failing otherwise
  - drop references to now-dead iconv
  - swap some `proc` for `Proc.new`
  - swap some `lambda` for `expect` in the spec suite; this is not
    merely cosmetic as the LocalJumpError tests were failing without it
  - drop some "should" from `it` descriptions in the spec suite
  - loosen brittle TypeError assertions in spec suite which would
    otherwise have failed (the error message has changed since 1.9.x)
  - always use a space after a comma separating arguments
  - replace `not` and `or` with `!` and `!!`
  - use `__dir__` in lieu of `File.dirname(__FILE__)`
  - update copyright year ranges in touched files
  - update docs prior to release

Signed-off-by: Wincent Colaiuta <win@wincent.com>