Added separate integration specs.

* This speeds up the normal `rake spec` test suite, and reduces the need
  to install `javac` or `mingw64-gcc` just to run the test suite.
* The non-integration tests can be ran via `rake spec`.
* The integration tests can be ran via `rake spec:integration`.
* The entire test suite can be ran via `rake test`.

Rakefile

@@ -23,7 +23,14 @@ Gem::Tasks.new(sign: {checksum: true, pgp: true})
 
 require 'rspec/core/rake_task'
 RSpec::Core::RakeTask.new
-task :test    => :spec
+
+namespace :spec do
+  RSpec::Core::RakeTask.new(:integration) do |t|
+    t.rspec_opts = '--tag integration'
+  end
+end
+
+task :test    => [:spec, 'spec:integration']
 task :default => :spec
 
 require 'yard'

spec/builtin/bin/unix/reverse_shell_spec.rb

@@ -58,7 +58,7 @@
     )
   end
 
-  describe "#build" do
+  describe "#build", :integration do
     before { subject.build }
 
     it "must set #payload" do

spec/builtin/bin/windows/reverse_shell_spec.rb

@@ -67,7 +67,7 @@
     )
   end
 
-  describe "#build" do
+  describe "#build", :integration do
     before { subject.build }
 
     it "must set #payload" do

spec/builtin/java/reverse_shell_spec.rb

@@ -26,7 +26,7 @@
     )
   end
 
-  describe "#build" do
+  describe "#build", :integration do
     before { subject.perform_build }
 
     it "must generate generate a new .java file containing the host and port" do

spec/spec_helper.rb

@@ -2,3 +2,7 @@
 require 'simplecov'
 
 SimpleCov.start
+
+RSpec.configure do |specs|
+  specs.filter_run_excluding :integration
+end