Backwards Compatible Dotenv::Environment

* Commit 7f82f73 introduced a backwards-incompatible change with
  the current API with respect to `Dotenv::Environment#intialize`
  by changing the method signature.
* Make the new parameter default to false to retain backwards
  compatibility.
* Similar changes were made for `Dotenv::Parser` in bkeepers#338.

SEE: bkeepers#323
SEE: bkeepers#338

lib/dotenv/environment.rb

@@ -4,12 +4,12 @@ module Dotenv
  class Environment < Hash
  attr_reader :filename
 
- def initialize(filename, is_load)
+ def initialize(filename, is_load = false)
  @filename = filename
  load(is_load)
  end
 
- def load(is_load)
+ def load(is_load = false)
  update Parser.call(read, is_load)
  end