* build: remove env.sh
This removes the dirty symlink-to-self hack we've had for years. The
script was added to enable building without GOPATH and did that job
reliably for all this time. We can remove the workaround because modern
Go supports building without GOPATH natively.
* Makefile: add GO111MODULE=on to environment
* build: bump PPAs to Go 1.13 (via longsleep), keep Trusty on 1.11
* travis, build, vendor: use own Go bundle for PPA builds
* travis, build, internal, vendor: smarter Go bundler, own untar
* build: updated ci-notes with new Go bundling, only make, don't test
* build: add support for different package and binary names
* build: bump up copyright date
* build: change default PackageName to empty string
* build, internal, swarm: enhance build/release process
* build: hack ethereum-swarm as a "depends" in deb package
* build/ci: remove redundant variables
* build, cmd, mobile, params, swarm: remove VERSION file; rename Version to VersionMeta;
* internal: remove VERSION() method which reads VERSION file
* build: fix VersionFilePath to Version
* Makefile: remove clean_go_build_cache.sh until it works
* Makefile: revert removal of clean_go_build_cache.sh
