Preparation for libsass3.2

[asottile]

This sets the submodule to 3.2.0-beta.2.

A few notable differences:

• Backtraces are improved
• image_path is gone (libsass no longer supports image-url function)
• sass_make_data_context takes ownership of the string (so we strdup)
• Output always ends in a \n now
• Improved sourcemap support
• compressed mode now outputs BOM instead of @charset

I think it's probably worth merging this? and then when 3.2 drops we should be able to just upgrade.

[coveralls]

Changes Unknown when pulling 179d4d8 on asottile:libsass_32 into * on dahlia:python*.

[dahlia]

Huge thanks for your effort as always. 😄 I just created a new branch named 0.7-maintenance as well. From now on master will be for the next major release, and 0.7-maintenance for minor release.