Enable fail_on_error

By setting `fail_on_error = false` we can easily overlook
some test failures.
https://travis-ci.org/treasure-data/presto-client-ruby/jobs/637287789?utm_medium=notification&utm_source=github_status

Rakefile

@@ -6,9 +6,7 @@ require 'rake/clean'
 
 require 'rspec/core/rake_task'
 
-RSpec::Core::RakeTask.new(:spec) do |t|
-  t.fail_on_error = false
-end
+RSpec::Core::RakeTask.new(:spec)
 
 task :default => [:spec, :build]