Merge master branch from rubygems/rubygems upstream.

* Enable Style/MethodDefParentheses in Rubocop
    https://github.com/rubygems/rubygems/pull/2478
  * Enable Style/MultilineIfThen in Rubocop
    https://github.com/rubygems/rubygems/pull/2479
  * Fix required_ruby_version with prereleases and improve error message
    https://github.com/rubygems/rubygems/pull/2344
  * Fix bundler rubygems binstub not properly looking for bundler
    https://github.com/rubygems/rubygems/pull/2426

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65904 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/rubygems/resolver/set.rb

@@ -31,7 +31,7 @@ class Gem::Resolver::Set
   # The find_all method must be implemented.  It returns all Resolver
   # Specification objects matching the given DependencyRequest +req+.
 
-  def find_all req
+  def find_all(req)
     raise NotImplementedError
   end
 
@@ -43,7 +43,7 @@ class Gem::Resolver::Set
   # When overridden, the #prefetch method should look up specifications
   # matching +reqs+.
 
-  def prefetch reqs
+  def prefetch(reqs)
   end
 
   ##