Support for rails 6.0.x

[akshaysasidrn]

• Adds gemfile for rails 6 and updates for Travis CI build
• Update migrations for differing Rails versions
• Removes code for unsupported Rails versions
• Bumps up rapidfire version for major release

[Skeyelab]

Eagerly awaiting this to be merged in. I have a rails 6 app that could really benefit from this.

[akshaysasidrn]

@iffyuva shall we get this merged in?

[iffyuva]

@aksh93 merge and release this please.