ROLLBACK PR 470 to original SCREENSHOT_PATH behavior

The 'fix' did not work because I did not understand the intended
behavior of SCREENSHOT_PATH.

https://github.com/calabash/calabash-ios/pull/470
@todo re: changing behavior of SCREENSHOT_PATH

The behavior of this variable is conflated with the idea of a
'prefix'; it should just be a path to a directory.
correct the env. variable docs for SCREENSHOT_PATH
Merge pull request #474 from calabash/feature/rollback-PR470-screenshot-path-adjustment

rollback PR 470 changes re: SCREENSHOT_PATH