Read Go version from Dockerfile in order to keep things 'DRY'.

vagrant-common.rb

@@ -1,4 +1,8 @@
-$go_version = "1.5.2"
+$go_regexp = /FROM golang:(\S*).*?/
+$go_version = File.readlines("build/Dockerfile").first { |line| line.match($go_regexp) }.match($go_regexp).captures.first
+if $go_version.nil?
+  raise ArgumentError.new("Failed to read Go version from Dockerfile.")
+end
 
 $go_path = "/usr/local/go/bin"