TLS and SLURM configuration

crew.cluster 0.1.2

• Require crew >= 0.5.0.
• Add a new slurm_time_minutes for SLURM wall time (#1, @cfljam).
• Enable TLS.
• Use MB format for memory in SLURM launcher (#22, @kendonB).
• Use launch_max and crew_tls() from crew 0.5.0.