[Rake] Check required binaries

hg (and git, for that matter) is required to run the install_subspecs spec. As a result, this
commit makes the Rake tasks die quickly if it is not installed.

Rakefile

+REQUIRED_COMMANDS = %w[ git hg ]
+
+# Task check_requirements
+#-----------------------------------------------------------------------------#
+desc 'Verifies that the required third-party commands are available'
+task :check_requirements do
+  has_all_requirements = REQUIRED_COMMANDS.inject(true) do |has_requirements, required_command|
+    result = has_required_binary?(required_command)
+    puts red("Missing required command: #{required_command}. You may need to install it.") unless result
+    has_requirements && result
+  end
+  raise unless has_all_requirements
+end
+
 # Bootstrap task
 #-----------------------------------------------------------------------------#
 
 desc "Initializes your working copy to run the specs"
-task :bootstrap, :use_bundle_dir? do |t, args|
+task :bootstrap, [:use_bundle_dir?] => [:check_requirements] do |t, args|
   title "Environment bootstrap"
 
   puts "Updating submodules"
@@ -113,7 +127,7 @@ begin
     #--------------------------------------#
 
     desc "Run the integration spec"
-    task :integration do
+    task :integration => :check_requirements do
       unless File.exists?('spec/cocoapods-integration-specs')
         $stderr.puts red("Integration files not checked out. Run `rake bootstrap`")
         exit 1
@@ -128,7 +142,7 @@ begin
     # The specs helper interfere with the integration 2 specs and thus they need
     # to be run separately.
     #
-    task :all => :unpack_fixture_tarballs do
+    task :all => [:unpack_fixture_tarballs, :check_requirements] do
       ENV['GENERATE_COVERAGE'] = 'true'
       puts "\033[0;32mUsing #{`ruby --version`}\033[0m"
 
@@ -300,3 +314,9 @@ end
 def red(string)
   "\033[0;31m#{string}\e[0m"
 end
+
+def has_required_binary?(name)
+  `which #{name}`
+  $?.success?
+end
+