*: check go version before building

Add a script to check the Go version before building dex. This
gives a nice error message rather than just failing to compile.

With changes:

    $ go version
    go version go1.6.4 linux/amd64
    $ make
    ERROR: dex requires Go version 1.7+. Please update your Go installation: https://golang.org/dl/
    Makefile:93: recipe for target 'check-go-version' failed
    make: *** [check-go-version] Error 2

Checks only added for building the actual binary, not tests, since
this is aimed at users just starting off with the project.

Makefile

@@ -25,10 +25,10 @@ LD_FLAGS="-w -X $(REPO_PATH)/version.Version=$(VERSION)"
 
 build: bin/dex bin/example-app
 
-bin/dex: FORCE
+bin/dex: check-go-version
 	@go install -v -ldflags $(LD_FLAGS) $(REPO_PATH)/cmd/dex
 
-bin/example-app: FORCE
+bin/example-app: check-go-version
 	@go install -v -ldflags $(LD_FLAGS) $(REPO_PATH)/cmd/example-app
 
 .PHONY: release-binary
@@ -88,6 +88,10 @@ bin/protoc: scripts/get-protoc
 bin/protoc-gen-go:
 	@go install -v $(REPO_PATH)/vendor/github.com/golang/protobuf/protoc-gen-go
 
+.PHONY: check-go-version
+check-go-version:
+	@./scripts/check-go-version
+
 clean: clean-release
 	@rm -rf bin/