gitignore: don't use bare leading slash

The current .gitignore does not work correctly with my version of git (1.8.4), as some test paths are not getting filtered out:

```
# On branch gitignore
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#	test/.libs/
#	test/run-tests
nothing added to commit but untracked files present (use "git add" to track)
```

Removing the leading slash seems to always filter correctly, regardless of version