The codegen routines for ksonnet-lib depend on some Go 1.8 features
(e.g., `sort.Slice`). Once we vendor the dependency, this will cause the
build to fail perpetually for Go 1.7.

This commit therefore drops support for 1.7, making 1.8 the line of
supportability. This decision was made together with @anguslees, in
slack PMs.

Currently contains basic "update" and "delete" tests.
More cases will be added in followup patches.

Travis' deploy.on golang version check seems to be surprising.  In
particular `deploy.on.go=1.8.x` will *not* match a build performed
with `go=['1.8.x']`.

This change just gives up and uses an explicit bash conditional.
Uglier, but fewer surprises.

Frequent `make test` crashes on (default) xcode7.3 in jsonnet_cgo. Use
a different/newer build environment.

Prefer `1.8.x` rather than `1.8`.  In particular `deploy.on.go=1.8` does
not match (eg) `1.8.3` :(

Native function tests crash on golang 1.7/osx.  Unclear if this is a
golang/cgo bug on osx, or jsonnet_cgo is doing something incorrectly.
Not critical, provided the golang 1.8 builds continue to work.