Add check-vendor target to Makefile
Add check-vendor target to Makefile

The check-vendor target will exit with an error if any files in vendor/
differ from what's checked into git.

govendor checksums can get out of sync if extraneous files are in the
local, third-party repo (see kardianos/govendor#129).  govendor also
updates the vendor.json upon "sync."  We can use that quirk to detect
out of sync checksums in CI, so I've added check-vendor to the check
target dependencies.

anthonyfok/hugo,fedelibre/hugo,mdhender/hugo,fedelibre/hugo,mdhender/hugo,digitalcraftsman/hugo,gohugoio/hugo,fedelibre/hugo,mitchchn/hugo,gohugoio/hugo,gohugoio/hugo,fedelibre/hugo,mdhender/hugo,mdhender/hugo,anthonyfok/hugo,mitchchn/hugo,anthonyfok/hugo,digitalcraftsman/hugo