travis: Adding update as travis suggested.
travis: Adding update as travis suggested.

```
Running apt-get update by default has been disabled.
You can opt into running apt-get update by setting this in your .travis.yml file:
  addons:
    apt:
      update: true
```

mithro/HDMI2USB-litex-firmware,mithro/HDMI2USB-litex-firmware,mithro/HDMI2USB-litex-firmware,mithro/HDMI2USB-litex-firmware