Revert "A fix for #613"

This reverts commit 6e591803491a1f88ce86594c901d4810b9b2c245.

The lock should be in the compiled code, and the lock needs to be
global to prevent all users of $stdout not just other thread in the
filter.
Use exclusive lock in :ruby filter

The :ruby filter redirects $stdout, which causes problems in multi-
threaded environments.

Wrap code generated by :ruby filter in `Thread.exclusive` block.

Also use ensure block to make sure $stdout is reset correctly if an
exception is raised.
Update CHANGELOG with changes to :ruby filter
Update CHANGELOG

Add details of --autoclose command line option.
Don't use Rails master gemfile in 1.8.7

Rails 2 is no longer supported, so remove check in gemfiles, but add
check to ignore Rails master when testing in 1.8.7.
Rails 4 requires Ruby 1.9.3+

Fix Rakefile to correctly choose when to ignore Rails master.
Render ; instead of \n when multiple blocks end

That way the source line number matches the haml input.
Remove unused Dumpable class from util_test

A remnant from Sass, no longer used.
Fix version detection code for Rails 3.2.3+.
Add tests for html_escape and escape_once
Do not use silence_warnings in html_escape

silence_warnings involves reassigning $stderr, which is dangerous in
threaded environments. As a step towards removing it, remove its use
in html_escape and escape_once.
Encode regexes to match text rather than force text to BINARY.
Revert "Use exclusive lock in :ruby filter"

This reverts commit 68977f3368941a5569c9345d3e9012f8184b0419.

Thread.exclusive isn't available in 1.8.7, and doesn't prevent other
threads running in 1.9 anyway.
Don't redirect $stdout in :ruby filter.

Redirecting $stdout isn't thread safe.

Provide `haml_io`, an IO object that writes to the buffer.
Merge pull request #620 from haml/html_escape_warnings

Remove use of `silence_warnings` in `html_escape`
Don't assert over specific error message

This triggers a failure on Rubinius otherwise:

https://travis-ci.org/haml/haml/jobs/4429546
Enabled Rails asset helpers inside the Sass and Scss filters.

Closes #467
Bumped version
Merge branch 'master' into 3-2-stable