Update step

[kdobmayer]

Hi @deanWombourne,

Thanks for the new Step! I created a little update to follow shell scripting guide lines.

• Updated Shebang for portability (bash doesn't live in /bin on all systems)
• A top-level comment with a description of its contents
• Install npm only if not previously installed
• There is no need for set -o pipefail if not using any pipes
• In case of command substitution $(...) is preferred over backticks
• Double quote variables to avoid world splitting