ESLint conversion from JSHint

Fixes: https://github.com/sass/node-sass/pull/1631
Fixes: https://github.com/electron/electron/issues/6462
Broken-By: https://github.com/sass/node-sass/pull/1101
Fix-Provided-By: https://github.com/sass/node-sass/pull/1631

http://eslint.org/docs/rules/no-unused-vars

http://eslint.org/docs/rules/no-unused-expressions

Suppressed the ones coming from the Sass Types

Newer rule system and auto-fixing errors

We don't use gitter anymore,

The options object passed into `render` and `renderSync` is only
shallow copied. This is an issue when nested objects and arrays like
custom importer and functions. In the case of custom function we
wrap the provided function. This wrapped version replaces the provided
one which can then leak back into the calling code. This is an issue
if `render*` is called in a loop like in gulp-sass.

Fixes #1168

Chore: Fix Coveralls conditional for Travis-CI

Broken when switching from the old matrix to the built in node_js
version

* In watch mode, play an audible cue when there is an error.

* Added error-bell option.

* Added error-bell option to documentation.

Both packages have been merged and cross-spawn is now
doing the same as cross-spawn-async before

Currently we build the Sass import graph when the CLI watcher is
started. However the graph is not update updated when files are
added or deleted. The latest is a big deal but the former results
in new files not triggered rebuilds when they're changed. The only
way to currently resolve this is to restart the CLI watcher.

This patch rebuilds the Sass import graph when files are added or
deleted to the watch always works as expected.

Fixes #1538

Renamed parameters to be a little clearer, and marked them public since
they are being exported

For the lazy

The support was added in sass/node-sass#1499

Currently it's a bit of an ordeal trying to get the atomic puush
required for AppVeyor to register a commit on the release branch
and also a tag. Looks like AppVeyor expects you to do release from
the master branch.

Inspired by @nschonni work in #1518 in an attempt to work around
this issue this PR instructs AppVeyor to ignore tags on non-release
branches.

Fingers crossed this makes like easier.

Closes #1518

Closes #1486

Node 6.x has trouble including modules
from MS-DOS drives created with subst.exe.

Since we use use subst only to set
symbols paths in the PDB file, let's
revert to the C: drive when doing tests.

Workaround for: https://github.com/nodejs/node/issues/6500

Previous we manually installed NVM and did the version switching
ourselves. This was required because Trvais didn't support NVM
on OSX builds. This has since [been fixed][1] so lets see how it
goes.

[1:][https://github.com/travis-ci/travis-ci/issues/2311]

This was previously attempted in #1413. Shortly after it's
release proxy users started experiencing installation issues so
this was reverted. It was later determined that #1458 was likely
at fault for the proxy issues.

Full credit for this patch goes to @delitescere.

I've also taken the liberty of cleaning the request config generation.

Fixes #1333

This was partially addressed by bumping NAN in #1496.

https://github.com/nodejs/node/issues/6216
https://codereview.chromium.org/1839983002/patch/1/10002

Fixes #1484

This removes the deprecation warning in Node 6

The human friendly name for an unknown platform, arch or runtime
is currently `false`. With this patch the user is better informed
as to why their environment is unsupported. The raw value is also
output to ease with bug reports.

Closes #1487

v3.6.0: Bump LibSass to 3.3.6

This reverts commit c2d24c23.

Unfortunately got does not have good [proxy support][1] which
was missed during review.

Fixes #1458

[1]: https://github.com/sindresorhus/got/issues/79