Test performance: define a lower post max length

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.