Fix for version.sh when libvpx is checked out as git submodule

When using git submodules, .git may be a file instead of a directory.
The -d test was failing in that case; switched to -e.

BUG=https://bugs.chromium.org/p/webm/issues/detail?id=1215

Change-Id: Iedf0e92bfeb003b28a415945dc729e6ce58c4fe4
1 file changed