Make escape_once respect hexadecimal references

The regex for escape once recognises decimal numeric character
references and character entity references, but not hexadecimal
numeric character references.

Add hexadecimal references to the regex.
Add test for attribute methods

When an attribute method (or literal nested hash) is used in an old
style attribute hash and there is also a (non-static) new style hash
there is an error. The generated code looks something like:

    foo.merge(method_that_returns_hash, :foo => 'bar')

Add a test to expose this.
Fix for attribute methods

See 0ea157839f7fbc98d058cf4af2dac0d007ea4416

The call to `merge` isn't necessary since the hashes are merged in the
call to `Buffer#attributes` with `merge_attrs`

This commit can result in different behaviour in certain case, since
the hashes are being merged with `merge_attrs` which handles `class` and
`id` attributes. Previously the value from the 'old style' would replace
the value from the 'new style`

e.g.

    %p.the_class{:class => old_attrs}(class = new_attrs)

(where `old_attrs` and `new_attrs` are methods that return 'old' or
'new')

Previously this would result in:

    <p class='old the_class'></p>

with this commit the result is:

    <p class='new old the_class'></p>
Test in CI against Rails 4 beta and Ruby 2.0
Fix textareas on Rails 4
DRY up test_area_test
Use explicit source URLs in Gemfiles
Depend on Bundler 1.3 with Rails 4
Try to ensure Travis uses the latest bundler
Just use "jruby" and "rbx-19mode" in CI; don't allow failures
Updated changelog
Merge pull request #637 from haml/attribute-methods

Attribute methods
Merge pull request #633 from haml/escape-once-hexadecimal

Make escape_once respect hexadecimal references
Test in CI against Rails 4 beta and Ruby 2.0
Fix textareas on Rails 4
DRY up test_area_test
Use explicit source URLs in Gemfiles
Depend on Bundler 1.3 with Rails 4
Try to ensure Travis uses the latest bundler
Just use "jruby" and "rbx-19mode" in CI; don't allow failures