* travis.yml: work around Go 1.9.4 issue

* travis: workaround the workaround

This works around travis-ci/travis-ci#8703.

This avoids setting CC for the go run invocation, which fails on go1.10.

Also remove installation steps for fuse and golang.org/x/tools/cmd/cover 
because they're not required anymore.

* build: [finishes #15372] implements generalized linter and travis job

* .travis, build: minor polishes, disable deadcode

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