Go back to previous dir after compiling

There is a `process.chdir` happening in `recursiveModuleCompileToSingleFile` that should be reverted once the operation has been completed.

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/1290

Closes #1290.
Revert "unit test"

This reverts commit 089ecb00babfc69c36f00f0daa6bb94d825c90bb.
added test for working dir change