Restore POSIX shell compatibility

When `npmrc` is executed with `dash` it throws the following error:

```
./npmrc: 6: [: unexpected operator
```

This is due to 0c0b79f introducing string comparison using double equal signs which [doesn’t work in POSIX shells](http://stackoverflow.com/questions/1089813/bash-dash-and-string-comparison). This commit restores POSIX shell compatibility by using single equals signs.