Run rvm use when sw_vers is available

lib/travis/build/appliances.rb

@@ -16,6 +16,7 @@
 require 'travis/build/appliances/fix_resolv_conf'
 require 'travis/build/appliances/home_paths'
 require 'travis/build/appliances/put_localhost_first'
+require 'travis/build/appliances/rvm_use'
 require 'travis/build/appliances/services'
 require 'travis/build/appliances/show_system_info'
 require 'travis/build/appliances/validate'

lib/travis/build/appliances/rvm_use.rb

@@ -0,0 +1,15 @@
+require 'travis/build/appliances/base'
+
+module Travis
+  module Build
+    module Appliances
+      class RvmUse < Base
+        def apply
+          sh.if "$(command -v sw_vers)" do
+            sh.cmd "rvm use", echo: true
+          end
+        end
+      end
+    end
+  end
+end

lib/travis/build/script.rb

@@ -157,6 +157,7 @@ def configure
           apply :disable_ssh_roaming
           apply :debug_tools
           apply :npm_registry
+          apply :rvm_use
         end
 
         def checkout

spec/build/script/shared/appliances/rvm_use.rb

@@ -0,0 +1,6 @@
+shared_examples_for 'rvm use' do
+  let(:sexp) { sexp_filter(subject, [:if, '$(command -v sw_vers)']) }
+  it 'runs "rvm use"' do
+    expect(sexp).to include_sexp [:cmd, "rvm use", echo: true]
+  end
+end

spec/build/script/shared/script.rb

@@ -56,6 +56,7 @@
   it_behaves_like 'build script stages'
   it_behaves_like 'npm registry override'
   it_behaves_like 'update libc6'
+  it_behaves_like 'rvm use'
 
   it 'calls travis_result' do
     should include_sexp [:raw, 'travis_result $?']