i3status-rust: update to 0.20.3.

[vokar]

0.20.3 contains several useful fixes, such as the NetworkManager block working for new versions of NM. For more details see https://github.com/greshake/i3status-rust/blob/master/NEWS.md#i3status-rust-0203

General

• This is a new package and it conforms to the quality requirements

Have the results of the proposed changes been tested?

• I use the packages affected by the proposed changes on a regular basis and confirm this PR works for me
• I generally don't use the affected packages but briefly tested this PR

[ericonr]

@jcgruenhage ok?

[jcgruenhage]

yep, looks good