Update vendored go-getter

[apparentlymart]

New features of interest in this update:

• Support for .tar.xz archives for module sources.
• Git sources will automatically recursively initialize submodules.

This fixes #15889.

[catsby]

This seems legit 👍

[apparentlymart]

$ TF_ACC=1 go test -v ./command/e2etest
=== RUN   TestInitProviders
=== RUN   TestPrimarySeparatePlan
=== RUN   TestVersion
--- PASS: TestVersion (0.02s)
--- PASS: TestInitProviders (1.63s)
2017/08/24 11:08:25 [TRACE] Preserving existing state lineage "46037fb4-3b11-4b4a-b251-f18aa2fe4d4d"
2017/08/24 11:08:25 [TRACE] Preserving existing state lineage "46037fb4-3b11-4b4a-b251-f18aa2fe4d4d"
2017/08/24 11:08:25 [TRACE] Preserving existing state lineage "46037fb4-3b11-4b4a-b251-f18aa2fe4d4d"
2017/08/24 11:08:25 [TRACE] Preserving existing state lineage "46037fb4-3b11-4b4a-b251-f18aa2fe4d4d"
--- PASS: TestPrimarySeparatePlan (3.57s)
PASS
ok  	github.com/hashicorp/terraform/command/e2etest	26.014s

(Running this because the TestInitProviders function here exercises the path that uses go-getter to download and install plugins.)

[ghost]

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