Prepare for upgrade to rspec 3.0.0.rc1

* remove rspec-instafail, it is not compatible
* convert some uses of `should`'s to `expect`

Signed-off-by: John Foley <jfoley@pivotallabs.com>
Bump serverspec and specinfra to latest

Signed-off-by: John Foley <jfoley@pivotallabs.com>