build: Update submodules
Merge pull request #3317 from pcwalton/update-submodules

build: Update submodules
Don't fail converting invalid UTF8 when fetching JS source. Fixes #3302.
Fix incorrect path to about:failure source.
Stop messing with the case of the attribute name in AttributeHandlers::get_attribute.

This fixes a bug where GetAttributeNS would incorrectly match lower-case
attributes when called with an upper-case argument.
Merge pull request #3323 from Ms2ger/get_attribute

Stop messing with the case of the attribute name in AttributeHandlers::get_attribute; r=Manishearth
Merge pull request #3325 from jdm/jsdecode

Don't fail converting invalid UTF8 when fetching JS source. Fixes #3302.
Reinstate errors for unused variables and imports.
Remove obsolete FIXME comment.
Call from_base64 on a vector slice rather than a string slice (fixes #2877).
Merge pull request #3329 from Ms2ger/2877-base64

Call from_base64 on a vector slice rather than a string slice (fixes #2877).
Merge pull request #3328 from Ms2ger/fixme-customevent

Remove obsolete FIXME comment.
Merge pull request #3327 from metajack/cargo-profiles

Reinstate errors for unused variables and imports.
Fix broken build after denying unused imports.
Updates tests README.md for mach
Merge pull request #3282 from gilles-leblanc/tests-readme

Updates tests README.md for mach; r=Manishearth
Add a `mach rustc` command
Merge pull request #3334 from mbrubeck/mach-rustc

Add a `mach rustc` command