Use let instead of before blocks

See:

* Better Specs | Use let and let!http://betterspecs.org/#let
* StackOverflow | When to use rspec let()? -
http://stackoverflow.com/questions/5359558/when-to-use-rspec-let/5359979
#5359979
* RSpec Let and let! -
https://www.relishapp.com/rspec/rspec-core/v/2-11/docs/helper-methods/le
t-and-let
Install Faker gem
Refactor frequently used data hashes
Merge branch 'master' of github.com:itsmrwave/pesapal-gem

* 'master' of github.com:itsmrwave/pesapal-gem:
  Refactor frequently used data hashes
  Install Faker gem
  Use let instead of before blocks