Allow Airbrussh to be used without rake installed (#127)

We only actually need rake if the `monkey_patch_rake` setting is true.
By default it is false, so airbrussh should be usable without rake.

However we were running `require "rake"` regardless of this config,
which meant that in practice you would get a `LoadError` if rake is
unavailable.

Fix by running `require "rake"` only if `monkey_patch_rake` is true.

Fixes #126

lib/airbrussh/rake/context.rb

@@ -1,5 +1,3 @@
-require "rake"
-
 module Airbrussh
   module Rake
     # Maintains information about what Rake task is currently being invoked,
@@ -53,6 +51,7 @@ class << self
         attr_accessor :current_task_name
 
         def install_monkey_patch
+          require "rake"
           return if ::Rake::Task.instance_methods.include?(:_airbrussh_execute)
 
           ::Rake::Task.class_exec do