Performance: reduce max length post model for testing purpose

Hi there,

Just a little modification to improve the execution speed of the slowest test of the suite. 

Benchmark on my machine: 

Before 
==================
[vagrant@precise32:/vagrant (master)]$ rspec spec/models/post_spec.rb:20
Run options: include {:locations=>{"./spec/models/post_spec.rb"=>[20]}}
.

Finished in 8.45 seconds
1 example, 0 failures
==================

After 
==================
[vagrant@precise32:/vagrant (tests_performance)]$ rspec spec/models/post_spec.rb:20
Run options: include {:locations=>{"./spec/models/post_spec.rb"=>[20]}}
.

Finished in 1.08 seconds
1 example, 0 failures
==================

Thanks.