Improve test suite performance with aruba wrapper:

* Implement ArubaProgramWrapper in cucumber env;
* Tag a few features to not use the wrapper as aruba doesn't support yet
  interactive testing with in process mode, and reported program name
  will not be the correct one;
* Assign required standard streams to CLI and Env classes.

features/cli_usage.feature

@@ -1,5 +1,6 @@
 Feature: CLI usage
 
+  @exec
   Scenario: prints the usage when an argument is missing
     When I run `producer`
     Then the exit status must be 64

features/recipe_ask.feature

@@ -1,5 +1,6 @@
 Feature: `ask' recipe keyword
 
+  @exec
   Scenario: prompts user with a list of choices on standard output
     Given a recipe with:
       """

features/support/env.rb

@@ -1,2 +1,34 @@
 require 'aruba/cucumber'
+require 'aruba/in_process'
 require 'cucumber/sshd/cucumber'
+require 'producer/core'
+
+
+class ArubaProgramWrapper
+  def initialize(argv, stdin = $stdin, stdout = $stdout, stderr = $stderr,
+    kernel = Kernel)
+    @argv   = argv
+    @stdin  = stdin
+    @stdout = stdout
+    @stderr = stderr
+    @kernel = kernel
+  end
+
+  def execute!
+    Producer::Core::CLI.run!(
+      @argv.dup, stdin: @stdin, stdout: @stdout, stderr: @stderr
+    )
+  rescue SystemExit => e
+    @kernel.exit(e.status)
+  end
+end
+
+
+Before('@exec') do
+  Aruba.process = Aruba::SpawnProcess
+end
+
+Before('~@exec') do
+  Aruba::InProcess.main_class = ArubaProgramWrapper
+  Aruba.process = Aruba::InProcess
+end