Remove outdated items

[nutjob4life]

Merge this pull request to remove several outdated items in the top-level README.md file.

Specifically, it:

• Removes references to build_labcas.sh which no longer exists (see Support mock SSO service jpl-labcas/backend#16 (comment))
• Removes requirement for bash (since build_labcas.sh is gone, but this should be readily available on any Unix-like system)
• Replaces reference to script with Docker Comp
• Removes configuration for ip_address_placeholder (no longer appears)