Update MetaSnk.yaml, no includes specifics channels where to look for...

Update MetaSnk.yaml, no includes specifics channels where to look for dependencies. Fixes error due to unsolved environment when looking for snakemake.

envs/MetaSnk.yaml

 name: MetaSnk
 channels:
+  - conda-forge
+  - bioconda
   - defaults
 dependencies:
   - snakemake=5.5.0