Clic on donate crashes lazydocker

**Describe the bug**
Clic on donate crashes lazydocker on Debian 10 server (probably when xdg-open is not installed)
`errors.errorString sh: 1: xdg-open: not found`

**To Reproduce**
1. Launch lazydocker from a server (debian 10)
2. Click on 'donate'
3. See error

**Expected behaviour**
if xdg-open is not installed, just print the donation url

**Desktop (please complete the following information):**
- OS: Debian 10 server x86_64
- Lazydocker Version: 0.8
- Date: 2020-02-02T00:22:48Z
- BuildSource: binaryRelease
- Commit: cea67bc570daaa757a886813ff3c2763189efef6

**Additional context**
```
*errors.errorString sh: 1: xdg-open: not found

/home/circleci/project/pkg/commands/os.go:120 (0x87acd8)
/home/circleci/project/pkg/commands/os.go:62 (0x87a3a3)
/home/circleci/project/pkg/commands/os.go:87 (0x87b081)
/home/circleci/project/pkg/commands/os.go:87 (0x87b066)
/home/circleci/project/pkg/gui/gui.go:343 (0x8879d0)
/home/circleci/project/vendor/github.com/jesseduffield/gocui/gui.go:834 (0x864e3d)
/home/circleci/project/vendor/github.com/jesseduffield/gocui/gui.go:834 (0x864e1c)
/home/circleci/project/vendor/github.com/jesseduffield/gocui/gui.go:792 (0x864b4c)
/home/circleci/project/vendor/github.com/jesseduffield/gocui/gui.go:485 (0x863472)
/home/circleci/project/vendor/github.com/jesseduffield/gocui/gui.go:483 (0x86344a)
/home/circleci/project/pkg/gui/gui.go:303 (0x8876a7)
/home/circleci/project/pkg/gui/subprocess.go:20 (0x898b8f)
/home/circleci/project/pkg/app/app.go:54 (0x8ad653)
/home/circleci/project/pkg/app/app.go:54 (0x8ad646)
/usr/local/go/src/runtime/proc.go:200 (0x42d79c)
/usr/local/go/src/runtime/asm_amd64.s:1337 (0x4585b1)
```