Enable the use of context in restic

Set up a cancelble context in global options, hook it into the ctrl-C handler
for proper cancel propegation.

Bump up minimal requirement for Go to version 1.7 in documentation
and test-build files.

Vagrantfile

@@ -1,7 +1,7 @@
 # -*- mode: ruby -*-
 # vi: set ft=ruby :
 
-GO_VERSION = '1.6'
+GO_VERSION = '1.7'
 
 def packages_freebsd
   return <<-EOF