Fixes `projectile-replace` directory selection with `helm-mode`

In the current version of projectile (0.11.0) and helm-mode (1.6.4), explicitly passing a directory into `projectile-replace` results in the following error:

    Opening input file: no such file or directory, /path/to/directorypath/to/file.el

Specifically, a `/` is missing between `/path/to/directory` and `path/to/file.el` because `helm-mode`'s directory selection strips the trailing slash before returning it. This patch makes `projectile-replace` robust against file selectors that do this.