Support setting services[].ulimits[] to unlimited (-1)

[cgzones]

The following ulimit compose syntax is currently not supported:

ulimits:
  memlock:
    soft: -1
    hard: -1

resulting in

Error: 
   0: error converting compose file
   1: error reading compose file
   2: File `compose.yml` is not a valid compose file
   3: services.foo.ulimits.memlock.soft: invalid type: integer `-1`, expected u64 at line 43 column 15

Location:
   src/cli/compose.rs:203

Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
Run with RUST_BACKTRACE=full to include source snippets.

[k9withabone]

Blocked on k9withabone/compose_spec_rs#31.