Update to use crystal 0.31.1

[lbarasti]

Summary of the changes

• Initialize .guardian.yml with shards install rather than crystal deps - the latter was removed a few versions ago
• Invoke OptionParser.parse rather than OptionParser.parse!, as the latter has been deprecated and throws a warning at compile time
• Include crystal field in shard.yml to indicate "the last known Crystal version that is capable to compile the Shard", as per the shards spec

Testing

• I verified that the code still compiles fine when running crystal build src/guardian.cr --release
• I tested the newly built guardian on a crystal project and verified that guardian --init produces the expected output.