Travis: fetch all symengine's pull requests
Travis: fetch all symengine's pull requests

That way the `symengine_version.txt` can reference a commit that is only
available in a symengine's pull request that is not merged yet. This allows to
test a proposed change and its Ruby wrappers.

rajithv/symengine.rb,rajithv/symengine.rb,rajithv/symengine.rb,isuruf/symengine.rb,abinashmeher999/symengine.rb,symengine/symengine.rb,abinashmeher999/symengine.rb,isuruf/symengine.rb,symengine/symengine.rb,abinashmeher999/symengine.rb,isuruf/symengine.rb,symengine/symengine.rb