Refactor @at-root to ensure that it works properly with a plain &.

This changes the structure of the intermediate trees a little. Instead
of having the RuleNode nesting resolved entirely in the Cssize
visitor, the selectors are now resolved in Perform, while the nesting
itself is removed in Cssize. This is necessary to make the resolved
selectors consistently available to script &.

Closes #957
Add delayed evaluation of arguments to support the if builtin.
Make the if() function work as expected.

Only evalaute the corrent argument corresponding to the state of the
conditional.
Rubocop 0.14.1 compatibility.

Pin Rubocop's version here so we don't get any more unexpected build
breaks.
Handle missing method for readonly environments.
Bubble unknown nested directives.
Add the ability to use @at-root to move out of directives.

Closes #774
Improve the error message when using sass --sourcemap --stdin.

Closes #846
Add a test for using @extend within @at-root.

Also fix a bug exposed by this test.
Support trailing commas in lists and maps.

This also adds the syntax (value,) for creating a singleton list.

Closes #917
Be more liberal in the allowed ordering of var args.

Keyword arguments may occur after the first "...".

Closes #926
Add a Map value helper.
Use value helpers in some functions that still weren't.

Closes #952
Make the gem autobuild server respect RC versions.
Fix build breakage.
Mark Rubinius as an allowed Travis failure.

Its filesystem code seems to be flaky enough that
UtilTest#test_atomic_writes{,_handles_exceptions} fails occasionally.
Make the first step towards new variable semantics.

Global variable assignment will now print a deprecation warning unless
it uses the "!global" flag. In a future release, it will assign to a
local variable instead.

Closes #473
Bump VERSION to 3.3.0.rc.1.