Replace rbx with ruby-head in Travis
Replace rbx with ruby-head in Travis

rbx is failing to install in travis as rvm wants to install via binary
and there isn't a binary install available. Swap for Ruby head instead
so we can test on Ruby 2.4

Signed-off-by: Tim Smith <764ef62106582a09ed09dfa0b6bff7c05fd7d1e4@chef.io>

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