Bug 1042996 - Always set an explicit version in evalInSandbox. r=gabor

If setVersion() is not invoked on compileOptions, it ends up with
JSVERSION_UNKNOWN, which invokes findVersion() on the JSContext, which does a
bunch of crazy hunting of previous scripted stack frames that we most certainly
don't want for sandboxes, which are supposed to be controlled environments.
Using a separate JSContext in evalInSandbox isolates us from these effects, so
once we stop doing that we need to be more explicit here.
Bug 1042996 - Remove the special JSContext for Sandboxes. r=bobowen,r=gabor