Get test suite green again, via minitest

Perhaps I'm missing something about the ruby version required or some other dependency, but when I pull from master and attempt to `bundle install` and `rake` I get a lot of errors:

```
Thu Sep 11 16:31:08 rigpa ~/github/stackprof
 (master u=) ▸ rake
cp test/test_middleware.rb tmp/x86_64-darwin13.0/stage/test/test_middleware.rb
install -c tmp/x86_64-darwin13.0/stackprof/2.1.2/stackprof.bundle lib/stackprof.bundle
cp tmp/x86_64-darwin13.0/stackprof/2.1.2/stackprof.bundle tmp/x86_64-darwin13.0/stage/lib/stackprof.bundle
Warning: you should require 'minitest/autorun' instead.
Warning: or add 'gem "minitest"' before 'require "minitest/autorun"'
From:
  /opt/rubies/2.1.2-github1/lib/ruby/2.1.0/test/unit.rb:1:in `<top (required)>'
  /Users/rick/github/stackprof/test/test_middleware.rb:4:in `<top (required)>'
  /opt/rubies/2.1.2-github1/lib/ruby/gems/2.1.0/gems/rake-10.3.2/lib/rake/rake_test_loader.rb:15:in `block in <main>'
  /opt/rubies/2.1.2-github1/lib/ruby/gems/2.1.0/gems/rake-10.3.2/lib/rake/rake_test_loader.rb:4:in `select'
  /opt/rubies/2.1.2-github1/lib/ruby/gems/2.1.0/gems/rake-10.3.2/lib/rake/rake_test_loader.rb:4:in `<main>'
MiniTest::Unit::TestCase is now Minitest::Test. From /opt/rubies/2.1.2-github1/lib/ruby/2.1.0/test/unit/testcase.rb:8:in `<module:Unit>'
/opt/rubies/2.1.2-github1/lib/ruby/2.1.0/test/unit.rb:676:in `<class:Runner>': undefined method `_run_suite' for class `Test::Unit::Runner' (NameError)
	from /opt/rubies/2.1.2-github1/lib/ruby/2.1.0/test/unit.rb:261:in `<module:Unit>'
	from /opt/rubies/2.1.2-github1/lib/ruby/2.1.0/test/unit.rb:15:in `<module:Test>'
	from /opt/rubies/2.1.2-github1/lib/ruby/2.1.0/test/unit.rb:7:in `<top (required)>'
	from /opt/rubies/2.1.2-github1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /opt/rubies/2.1.2-github1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /Users/rick/github/stackprof/test/test_middleware.rb:4:in `<top (required)>'
	from /opt/rubies/2.1.2-github1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /opt/rubies/2.1.2-github1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /opt/rubies/2.1.2-github1/lib/ruby/gems/2.1.0/gems/rake-10.3.2/lib/rake/rake_test_loader.rb:15:in `block in <main>'
	from /opt/rubies/2.1.2-github1/lib/ruby/gems/2.1.0/gems/rake-10.3.2/lib/rake/rake_test_loader.rb:4:in `select'
	from /opt/rubies/2.1.2-github1/lib/ruby/gems/2.1.0/gems/rake-10.3.2/lib/rake/rake_test_loader.rb:4:in `<main>'
rake aborted!
Command failed with status (1): [ruby -I"lib" -I"/opt/rubies/2.1.2-github1/lib/ruby/gems/2.1.0/gems/rake-10.3.2/lib" "/opt/rubies/2.1.2-github1/lib/ruby/gems/2.1.0/gems/rake-10.3.2/lib/rake/rake_test_loader.rb" "test/test_middleware.rb" "test/test_stackprof.rb" ]

Tasks: TOP => default => test
(See full trace by running task with --trace)
```

It looks like I'm being advised to bring things up to date with current `minitest`.  This PR does that.

/cc @tmm1 
do we need minitest/spec here, the spec syntax isn't being used in these tests