[CAT-156] Handle missing H5AD files, bump travis CI env version

[NickAkhmetov]

This PR makes the scvelo_annotated.h5ad file optional by continuing iteration over the file name list if the file does not exist.

In order to fix CI, the travis config has been set to use a newer version of Ubuntu (focal), as the default (xenial) shipped with an outdated version of openssl which caused urllib3 to throw errors.

[NickAkhmetov]

I rolled back my previous attempt to solve CI build issues in portal-containers:

• urllib broke in CI due to old version of openssl
• I upgraded miniconda to 4.12.0 to get a newer version of openssl
• This led to h5py failing due to libh5so being missing
  • tried installing corresponding -dev package from apt, this did not fix it
  • updated h5py to 3.10 from 2.10 (which was the last minor version of 2)
  • this got past the install stage, but failed when running, as attempting to write the test anndata file failed with TypeError: expected unicode string, found b'TCG'
• Tried updating other dependencies (anndata, zarr, etc) but this led to further errors

I did figure out a safer way to set marker genes without triggering pandas warnings about setting data on a copy:

 adata.obs.loc[adata.obs["leiden"] == cluster, f"marker_gene_{str(i)}"] = marker_gene

I've rolled back to before the miniconda change and will try tackling this issue without upgrading the Python version.