restore compat with previous `require 'jruby/rack/version'`
version bump to 1.1.13.1
re-arrange targets - make sure vendor/rack is always in the jar
fill-in History.txt for 1.1.13.1
fix warning in Rack::Utils::Response#write_headers

jruby complains about

```
rack/utils.rb:440 warning: multiple values for a block parameter (2 for 1)
```

This had a warning

```
require 'rack/utils'

h = Rack::Utils::HeaderHash.new
h[:key] = :value

for k, v in h
  puts [k, v].inspect
end
```

But this not

```
h.each do |k, v|
  puts [k, v].inspect
end
```

So we should use each-like iterator instead of for-loop (follow the [ruby style guide](https://github.com/bbatsov/ruby-style-guide#syntax)).
bundle (latest) rack 1.4
prepare for 1.1.13.2
travis-ci test 1.1.13 branch
fill in History for 1.1.13.2
restore compatibility with JRuby < 1.6.7
older (rubygems) Gem API compatibility in specs
prepare for 1.1.13.3 (hopefully the last in the 1.1 line)
git push explicitly using current branch name
Rack 1.5.1 support for 'our' servlet session store (with backwards-compat)
support `rake gem` on RubyGems 2.0 (removed Gem::Builder)
use .equals() rather than == since the level string is not always the same object instance as defined by the RackLogger interface
kares' e-mail
no need for casts
gem.license = 'MIT' + rubyforge is getting deprecated [skip ci]

Conflicts:
	Rakefile
appraise latest bundles

Conflicts:
	Appraisals
	Gemfile
	Gemfile.lock
	gemfiles/rails23.gemfile
	gemfiles/rails23.gemfile.lock
	gemfiles/rails30.gemfile
	gemfiles/rails30.gemfile.lock
	gemfiles/rails31.gemfile
	gemfiles/rails31.gemfile.lock
	gemfiles/rails32.gemfile
	gemfiles/rails32.gemfile.lock
	gemfiles/rails40.gemfile
	gemfiles/rails40.gemfile.lock