update Dockerfile.minimal to use alpine 3.4 (and go 1.6) (#669)
update Dockerfile.minimal to use alpine 3.4 (and go 1.6) (#669)

The alpine-based minimal docker image has been broken since 0d7ae8e.
That commit removed Godep-based dependency management favor of go
1.5/1.6 vendoring. This commit moves the base alpine image forward to
3.4 which includes go 1.6.3. The resulting image of 0d7ae8e^ grows from
66 MB to 73 MB.
drcapulet/cfssl,drcapulet/cfssl,sfurman3/cfssl,geligaoli/cfssl,cloudflare/cfssl,cloudflare/cfssl,geligaoli/cfssl,sfurman3/cfssl,cloudflare/cfssl,geligaoli/cfssl,drcapulet/cfssl,cloudflare/cfssl,drcapulet/cfssl,sfurman3/cfssl,geligaoli/cfssl,sfurman3/cfssl,drcapulet/cfssl,sfurman3/cfssl,geligaoli/cfssl