Update reference to MiniTest -> Minitest

[Matt-Yorkley]

Fixes reference to Minitest namespace when instantiating a Minitest::Mock, as noted here.

  1) Error:
Rack::Cache::MetaStore::Disk#test_0023_purges meta store entry when the body does not exist:
NameError: uninitialized constant RackCacheMetaStoreImplementation::MiniTest
Did you mean?  Minitest
               Maxitest
    test/meta_store_test.rb:185:in `block (2 levels) in included'

[ioquatix]

Maxitest =)