Properly print out deprecation messages (#713)
Properly print out deprecation messages (#713)

`@column` in `Slim::Parser` is never assigned.
This commit make `#deprecated_syntax` to properly
print out where the column is.

For example:

```slim
p==' hello_world
```

prints

```
Deprecated syntax: =' for trailing whitespace is deprecated in favor of =>
  (__TEMPLATE__), Line 2, Column 4
    p=' hello_world
```
slim-template/slim,slim-template/slim,ericboehs/slim,slim-template/slim,ericboehs/slim