extra characters prepended to logs

After upgrading to 3.2.3 logspout started adding weird characters to the beginning of every line. For example

```
�time="2017-10-04T01:20:14Z" level=info msg="Done checkpointing in-memory metrics and chunks in 1.404599334s." source="persistence.go:665"
xtime="2017-10-04T01:25:14Z" level=info msg="Checkpointing in-memory metrics and chunks..." source="persistence.go:633"
�time="2017-10-04T01:25:16Z" level=info msg="Done checkpointing in-memory metrics and chunks in 1.506158531s." source="persistence.go:665"
xtime="2017-10-04T01:30:16Z" level=info msg="Checkpointing in-memory metrics and chunks..." source="persistence.go:633"
�time="2017-10-04T01:30:17Z" level=info msg="Done checkpointing in-memory metrics and chunks in 1.52509546s." source="persistence.go:665"
xtime="2017-10-04T01:35:17Z" level=info msg="Checkpointing in-memory metrics and chunks..." source="persistence.go:633"
�time="2017-10-04T01:35:19Z" level=info msg="Done checkpointing in-memory metrics and chunks in 1.49991571s." source="persistence.go:665"
xtime="2017-10-04T01:40:19Z" level=info msg="Checkpointing in-memory metrics and chunks..." source="persistence.go:633"
�time="2017-10-04T01:40:20Z" level=info msg="Done checkpointing in-memory metrics and chunks in 1.417177673s." source="persistence.go:665"
xtime="2017-10-04T01:45:20Z" level=info msg="Checkpointing in-memory metrics and chunks..." source="persistence.go:633"
�time="2017-10-04T01:45:22Z" level=info msg="Done checkpointing in-memory metrics and chunks in 1.480621256s." source="persistence.go:665"
xtime="2017-10-04T01:50:22Z" level=info msg="Checkpointing in-memory metrics and chunks..." source="persistence.go:633"
�time="2017-10-04T01:50:23Z" level=info msg="Done checkpointing in-memory metrics and chunks in 1.516549895s." source="persistence.go:665"
xtime="2017-10-04T01:55:23Z" level=info msg="Checkpointing in-memory metrics and chunks..." source="persistence.go:633"
�time="2017-10-04T01:55:25Z" level=info msg="Done checkpointing in-memory metrics and chunks in 1.527579546s." source="persistence.go:665"
xtime="2017-10-04T02:00:25Z" level=info msg="Checkpointing in-memory metrics and chunks..." source="persistence.go:633"
�time="2017-10-04T02:00:26Z" level=info msg="Done checkpointing in-memory metrics and chunks in 1.341651987s." source="persistence.go:665"
xtime="2017-10-04T02:05:26Z" level=info msg="Checkpointing in-memory metrics and chunks..." source="persistence.go:633"
�time="2017-10-04T02:05:28Z" level=info msg="Done checkpointing in-memory metrics and chunks in 1.350708813s." source="persistence.go:665"
xtime="2017-10-04T02:10:28Z" level=info msg="Checkpointing in-memory metrics and chunks..." source="persistence.go:633"
�time="2017-10-04T02:10:29Z" level=info msg="Done checkpointing in-memory metrics and chunks in 1.404537061s." source="persistence.go:665"
xtime="2017-10-04T02:15:29Z" level=info msg="Checkpointing in-memory metrics and chunks..." source="persistence.go:633"
�time="2017-10-04T02:15:30Z" level=info msg="Done checkpointing in-memory metrics and chunks in 1.383589267s." source="persistence.go:665"
```

Any idea how to get rid of this?