Work-around JRuby not being able to unset environment variable

MSP-9923

`ENV[variable] = nil` and `ENV.delete(variable)` does not work to unset
an environment variable in JRuby, so fake it by setting the environment
variable to an empty string instead, which works for the specific case
of TRAVIS_BRANCH usage.
Allow ruby-head to fail

MSP-9923

ruby-head build is currently failing due to

```
undefined method `unpack' for nil:NilClass (NoMethodError)
/home/travis/.rvm/gems/ruby-head/gems/cucumber-1.3.16/lib/cucumber/ast/step.rb:81:in
`text_length'
```