Revert "TF: build from tag"

This reverts commit 57cc1a9.

The referenced commit has now been released as 0.6.15 so we no longer need
to build from source.

    ➜  terraform git:(master) git tag --contains 9cb08cb229a821753bdba8a6fc8c010767dea6c1
    v0.6.15

I'll update the version number in a following commit.

terraform/Dockerfile

@@ -1,7 +1,8 @@
-FROM golang:1.6-alpine
+FROM alpine:3.3
 
 ENV PATH $PATH:/usr/local/bin
-ENV TERRAFORM_TAG 9cb08cb229a821753bdba8a6fc8c010767dea6c1
+ENV TERRAFORM_VER 0.6.13
+ENV TERRAFORM_ZIP terraform_${TERRAFORM_VER}_linux_amd64.zip
 ENV BINARY_WHITELIST \
   terraform \
   terraform-provider-aws \
@@ -13,16 +14,8 @@ ENV BINARY_WHITELIST \
   terraform-provisioner-local-exec \
   terraform-provisioner-remote-exec
 
-ENV PACKAGES "openssl openssh-client ca-certificates make bash git"
-
+RUN apk add --update openssl openssh-client ca-certificates && rm -rf /var/cache/apk/*
 RUN set -ex \
-    && apk add --update $PACKAGES \
-    && mkdir -p /go/src/github.com/hashicorp/ \
-    && cd /go/src/github.com/hashicorp/ \
-    && git clone https://github.com/hashicorp/terraform.git \
-    && cd terraform \
-    && git checkout $TERRAFORM_TAG \
-    && make dev \
-    && for file in $BINARY_WHITELIST ; do mv bin/$file /usr/local/bin ; done \
-    && rm -rf /go \
-    && rm -rf /var/cache/apk/*
+       && wget https://releases.hashicorp.com/terraform/${TERRAFORM_VER}/${TERRAFORM_ZIP} -O /tmp/${TERRAFORM_ZIP} \
+       && unzip /tmp/${TERRAFORM_ZIP} -d /usr/local/bin ${BINARY_WHITELIST} \
+       && rm /tmp/${TERRAFORM_ZIP}

terraform/terraform_spec.rb

@@ -38,7 +38,7 @@
   it "has the expected Terraform version" do
     expect(
       command("terraform version").stdout
-    ).to include("Terraform v0.6.15-dev")
+    ).to include("Terraform v0.6.13")
   end
 
   it "installs SSH" do