This commit adds a build step to travis to auto-delete unstable archives older than
14 days (our regular release schedule) from Azure via ci.go purge.

The commit also pulls in the latest Azure storage code, also switching over from
the old import path (github.com/Azure/azure-sdk-for-go) to the new split one
(github.com/Azure/azure-storage-go).

This ensures 'make test' finds all errors that remote CI would find.
Go 1.7 vet reports a false positive in package log, add a workaround.

We have decided to bump the requirement to Go 1.7 because it enables
subtests and allows dropping backwards-compatibility code. This is in
line with Go's support policy. Go 1.6 and earlier no longer receive
security updates.

The Android NDK was recently removed from gomobile, leading to our Android
builds failing. Starting from https://go-review.googlesource.com/#/c/35173/ ,
gomobile requires a locally installed NDK. This PR ensures that travis installs
that too before running the build steps.

* 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

This commit tweaks the debian packaging tool:

* All build environment metadata can now be overriden on the command
  line. This allows testing the CI build behaviour locally.
* -unstable packages now actually contain the binaries (oops)
* packages use Go 1.7 to build
* archiving is skipped for PR builds

The new build script, ci.go, replaces some of the older shell scripts.
ci.go can compile go-ethereum, run the tests, create release archives
and debian source packages.

Turns out we actually don't need it, USE_NUM_NONE works
because we also set USE_FIELD_INV_BUILTIN.