Remove build dependency on `godep`

Instead of using `godep path`, we can simply set the GOPATH directly to
point to the Godeps/_workspace. We can still use `godep` to manage the
dependencies on the Godeps/ tree, but we don't need to have it available
for straight builds from git.

v2: Rebased and moved to inside kube::setup_go_environment() function.

Tested:
- Built it without godep in $PATH:
  $ hack/build-go.sh
- Ran unit tests without godep in $PATH:
  $ hack/test-go.sh
- Retested after rebase.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Revert "change travis.yml to install godep"

This reverts commit 453c328fc9585a67f3524a84ce996f86a06ab80d.

Having `godep` installed is not necessarily for build & test, only to
manage the dependencies under the Godeps/ tree.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Merge pull request #1054 from filbranden/build_without_godep

Build without godep