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.

This should yield another 30-second speed up. Nobody looks
at the output of those anyway. We might want bring back gofmt later
and actually fail the build if source is not formatted.

This should decrease test runtime to about 30 seconds.