Use RSpec directly in Travis builds

We weren't specifying this explicitly, which meant it was invoking
`rspec` via `rake`. Simplify things by invoking `rspec` directly (and
also fix the borked build).

Change-Id: I8eebef7cb3301e13bab7c0ff7c385a1257202e4e
Reviewed-on: http://gerrit.causes.com/42639
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Tested-by: Shane da Silva <shane.dasilva@brigade.com>