Use new exec package

Fixes flynn/flynn#11
Fixes flynn/gitreceived#5
Fixes flynn/slugbuilder#13

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
Add Godeps as a dependency to Makefile

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
Don't log timestamps

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>