Add ldflags to 'go build' cmd to bring back version number

[dotcs]

In v0.7.1 the -ldflags "-X main.gronVersion=${VERSION}" argument to the go build cmd has been removed.

This PR reverts this change and re-introduces the flag to show the correct version string in the binary.

gron/main.go

Lines 47 to 49 in 13561bd

	// gronVersion stores the current gron version, set at build
	// time with the ldflags -X option
	var gronVersion = "dev"

Closes #97

[rperce]

@tomnomnom wanted to bring this one to your attention, if you missed it—it closes the bothersome (to me) #97 and should be pretty straightforward!