Fix rspec invokation in Rakefile due to changes in rspec 3.0.0

https://github.com/rspec/rspec-core/blob/26698e46d312802483cee456610508f92a3dde56/Changelog.md#300beta2--2014-02-17

Remove support for specifying the documentation formatter using 's',
'n', 'spec' or 'nested'. (Jon Rowe)
be_true and be_false are deprecated since rspec 3
stub_const should be used to stub constants
Treat symbols as metadata kdey with true values is default now
Updated gemspec
Refactored old syntax
Rescue exception fails otherwise
Allow failure for jruby-mode20 as the binary does not exist at
travis-ci
Wrong travis config format fixed
Merge branch 'feature/rspec_formatter_fix' of https://github.com/dg-vrnetze/aruba into dg-vrnetze-feature/rspec_formatter_fix

Conflicts:
	features/step_definitions/aruba_dev_steps.rb
Ignoring failures for jruby-20mode and jruby-21mode for now: https://github.com/travis-ci/travis-ci/issues/2432