use binstub if present

[bolandrm]

Hey,

This updates the rake task to use bin/rspec if it's present.

Please see previous discussion @ rspec/rspec-rails#914

[myronmarston]

The build is failing because the travis build winds up with bin/rspec so that file is present for the specs. To setup and run the build the same locally, do this:

$ bundle install --standalone --binstubs
$ bin/rspec

It would also be nice to having a spec that shows that rspec is used if bin/rspec is not present (even if the test suite was launched from bin/rspec -- you may have to fake it out) and another that shows that bin/rspec is used if present (even if it wasn't originally present when the test suite was launched).

[bolandrm]

@myronmarston Sure thing, I'll fix it up.

[fables-tales]

@bolandrm are you still up for doing this? It'd be cool to see this pull request closed! ✨

[xaviershay]

Closing, stale.

[myronmarston]

This has personally affected me so I've been planning to wrap this one up. Going to reopen so I don't forget.

[myronmarston]

Closing in favor of #1494.