Rust 1.33 warnings

Just making a note of this so i don't forget.
```warning: use of deprecated item 'std::error::Error::cause': replaced by Error::source, which can support downcasting
--> src/error.rs:116:48
|
116 | StorageError::CsvError(ref e) => e.cause(),
| ^^^^^

warning: use of deprecated item 'core::str::::trim_left': superseded by `trim_start`
--> src/telegraf.rs:98:68
|
98 | let counter_value = line[end_index.unwrap() + 1..].trim_left();
| ^^^^^^^^^

warning: use of deprecated item 'core::str::::trim_left': superseded by `trim_start`
--> src/telegraf.rs:98:68
|
98 | let counter_value = line[end_index.unwrap() + 1..].trim_left();
| ^^^^^^^^^

```