build, internal/build: misc improvements (#3229)

* travis.yml: don't create darwin/386 builds

* build: remove godep remains

* internal/build: improve archives

- enable compression for zip files
- don't write half-complete archives

* build: add -unstable to archive names

build/env.sh

@@ -19,7 +19,6 @@ if [ ! -L "$ethdir/go-ethereum" ]; then
 fi
 
 # Set up the environment to use the workspace.
-# Also add Godeps workspace so we build using canned dependencies.
 GOPATH="$workspace"
 export GOPATH