add environment variable to force a build

this can be used to workaround #517
Merge pull request #529 from vivid-planet/force-build

add environment variable to force a build
Cleanup tests
Merge pull request #533 from kevva/test-clean

Cleanup tests
Change `--output` to be a directory

This makes it easier to watch a lot of files for changes and compile them to a
desired directory rather than writing them in `process.cwd()`.
Merge pull request #534 from kevva/output-dir

Change `--output` to be a directory
CI: Use env.SKIP_SASS_BINARY_DOWNLOAD_FOR_CI.
Fixes #537.
Merge pull request #538 from am11/master

CI: Use env.SKIP_SASS_BINARY_DOWNLOAD_FOR_CI
Install: Removes excessive dependencies.
Code: Saves extra nesting.
Merge pull request #540 from am11/master

Install: Removes excessive dependencies
Binary: Removes v8 version from the dir name.
Install: Fixes error handling.
Install: Checks proxy from npm config.
Merge pull request #541 from am11/master

Binary: Removes v8 version from the dir name
workaround non-existing git submodules when installing from fork