Refactor install for #1477

[0xmachos]

This PR contains no functional changes.

Edit: This is for #1477

I've converted the script to use functions for each major step.

• bundle install: 1e3b254
• bundler detection: 2d9ba38
• Ruby version check: 8ea4e77
• OS detection: 5b2abf4
• Get user permission: 0c26203

Updated the info, warn and fatal functions to use colour output.

For consistency I wrapped variable expansion in curly braces. e.g "$var1" to "${var1}".

I replaced use of backticks for command output capture with braces. e.g. `whoami` to $(whoami).

I've tested this on Debian 9.3.

[bcoles]

Thanks

[bcoles]

Do you happen to own a Mac?