Define a setter for operation input variables

Until now the input variables defined with `processes` in a operation
were read-only. With this change it's possible to change the value of
the variables when executing the operation.
New RSpec version requirement: ~> 2.14

This fixes an issue that caused breaking specs on Ruby 2.1.1
Merges feature: Ruby 2.1.1 compatibility

Fixes an RSpec related issue that wrongly caused failing specs on Ruby
2.1.1.
Merges pull request #4: Setter for operation input
Updates version: v0.8.1 -> v0.9.0
Release v0.9.0