Fix bug in get_go_version_from_build_dockerfile.

Previously, the first line of the Dockerfile would have been selected, regardless of its value.
What we want is to actually select the first line matching the regex, and then extract the group corresponding to the Go version out of it.

vagrant-common.rb

@@ -18,7 +18,7 @@ end
 def get_go_version_from_build_dockerfile()
   go_regexp = /FROM golang:(\S*).*?/
   dockerfile_path = File.expand_path(File.join(File.dirname(__FILE__), 'build', 'Dockerfile'))
-  go_version = File.readlines(dockerfile_path).first { |line| line.match(go_regexp) }.match(go_regexp).captures.first
+  go_version = File.readlines(dockerfile_path).select { |line| line.match(go_regexp) }.first.match(go_regexp).captures.first
   if go_version.nil?
     raise ArgumentError.new("Failed to read Go version from Dockerfile.")
   end