Fix reading from stdin in Ruby 1.9.3

I think that `f.readlines.to_s` relies on an old behavior of the Array#to_s
method. In Ruby 1.9.3 the `f.readlines.to_s` construct gives me an array
representation instead of the expected concatenation of all lines.

Compare:

    irb> RUBY_VERSION
    => "1.9.3"
    irb> JSON.parse(StringIO.new("[\"asdf\\n\"]").readlines.to_s)
    => ["[\"asdf\\n\"]"]
    irb> JSON.parse(StringIO.new("[\"asdf\\n\"]").read)
    => ["asdf\n"]