Require 'tmpdir' from spec helper

* Fixes issue when using mktmpdir

http://apidock.com/ruby/Dir/mktmpdir/class

```
Failures:

  1) Fdoc::Cli#convert when the fdoc path does not exist raises an exception
     Failure/Error: let(:temporary_path) { Dir.mktmpdir("fdoc-cli") }
     NoMethodError:
       undefined method `mktmpdir' for Dir:Class
     # ./spec/fdoc/cli_spec.rb:5:in `block (2 levels) in <top (required)>'
     # ./spec/fdoc/cli_spec.rb:6:in `block (2 levels) in <top (required)>'
     # ./spec/fdoc/cli_spec.rb:13:in `block (2 levels) in <top (required)>'
     # ./spec/support/capture_helper.rb:6:in `capture'
     # ./spec/fdoc/cli_spec.rb:15:in `block (2 levels) in <top (required)>'
```