AppImage cannot run on Ubuntu bionic

Hi, thanks for making an AppImage. However, it refuses to run on Ubuntu bionic because it was built on a too new system:

```
libm.so.6: version `GLIBC_2.29' not found
```

Building it on an older system such as Ubuntu bionic should resolve this.
Thanks!

Reference: https://github.com/AppImage/appimage.github.io/pull/2649