Fix remaining Ruby warnings.

- spec/rspec/rails/minitest_lifecycle_adapter_spec.rb:
  > warning: assigned but unused variable - example

- spec/support/shared_examples.rb
  > warning: assigned but unused variable - spec_file

Ruby 1.9.3 seems to erroneously report the following warning:

- lib/rspec/rails/matchers/have_http_status.rb:
  > warning: assigned but unused variable - _ignored

This is logged on ruby-trunk: https://bugs.ruby-lang.org/issues/9943

Author: cupakromer

spec/rspec/rails/minitest_lifecycle_adapter_spec.rb

@@ -12,7 +12,7 @@
       define_method(:after_teardown)  { invocations << :after_teardown }
     end
 
-    example = example_group.example("foo") { invocations << :example }
+    example_group.example("foo") { invocations << :example }
     example_group.run(NullObject.new)
 
     expect(invocations).to eq([

spec/support/shared_examples.rb

@@ -12,7 +12,7 @@ def define_group_in(path, group_definition)
       Dir.mktmpdir("rspec-rails-app-root") do |dir|
         Dir.chdir(dir) do
           path.mkpath
-          spec_file = File.open(file, "w") do |f|
+          File.open(file, "w") do |f|
             f.write("$_new_group = #{group_definition}")
           end