This bumps all our packages to their latest versions, including some major versions (gulp-autoprefixer, gulp-svgmin, markdoown-it-anchor`).
Except for sass, all stable (not 0.x.x) packages now use ^ in their version strings, so that future installs can self-upgrade to new non-major versions.
I pinned the 0.x.x packages to specific versions just because things may break until they hit 1.0. I pinned sass to an exact version because sass > 1.32 prints some warnings that could confuse our contributors. I would prefer to clear these warnings by improving our Sass, then allow for versions of sass > 1.32.