Rspec tests should respect command line DEVELOPER_DIR settings

### Summary

https://travis-ci.org/calabash/run_loop/jobs/42737748

The tests are run in random order.  In example below from Travis CI, the DEVELOPER_DIR from a previous tests is interfering with the test simulator/binary compatibility examples.

```
Simulator/Binary Compatibility Check
  raises an error if libraries are not compatible
   xcrun: error: missing DEVELOPER_DIR path: /foo/bar

  targeting i386 simulator with binary that contains only a x86_64 slice (FAILED - 1)
  xcrun: error: missing DEVELOPER_DIR path: /foo/bar

  target only has arm slices (FAILED - 2)
    can launch if library is FAT
    xcrun: error: missing DEVELOPER_DIR path: /foo/bar

  targeting x86_64 simulator with binary that contains only a i386 slice (FAILED - 3)
  xcrun: error: missing DEVELOPER_DIR path: /foo/bar
   can launch if libraries are compatible (FAILED - 4)
```

### Acceptance

1. Rspec tests should not clobber DEVELOPER_DIR; it should be changed for the duration of the example and then changed back.