build: gzip tarballs

The tar command currently used to create the release `smocker.tar.gz`
files isn't compressing the archives, so add the `-z` in there. This
should reduce archive sizes from around 32mb to 10-15mb (compressing
a released tarball is usually 10mb, my local builds are 15mb,
so might vary depending on who runs the build) and not lead to
any confusion when folks run `tar -xzf smocker.tar.gz` and find
out the it's not compressed.

Current instructions using `tar xf` don't need to be updated since
that will detect if the archive is compressed.

Makefile

@@ -130,7 +130,7 @@ build/smocker.tar.gz:
 	$(MAKE) build
 	yarn install --frozen-lockfile --ignore-scripts
 	yarn build
-	cd build/; tar -cvf smocker.tar.gz *
+	cd build/; tar -czvf smocker.tar.gz *
 
 .PHONY: release
 release: build/smocker.tar.gz