[REQUEST] Make next_rails compatible with older ruby versions

[arielj]

Branch/Commit:

Latest version I guess, it can't be installed

Problem:

next_rails currently requires ruby 2.3.0 or newer, which makes it incompatible with old rails projects using older ruby versions

we should find the reason for this limitation and maybe add some note or workaround, or maybe we can split the gem into different gems with different ruby restrictions if maybe the ruby restrictions is there because one feature and not for everything

I will abide by the code of conduct

[etagwerker]

@arielj there is some information over here:

• 0a6ebaa
• Incompatible with Ruby < 2.3 clio/ten_years_rails#9

[arielj]

I was doing some tests and it seems that the ruby version limitation can be removed, next_rails only depends on colorize now when added as a gem and colorize has no ruby version restriction. I tried removing the gem restriction and I was able to install it in Ruby 2.1.9 with no errors.

[arielj]

I was able to add support from Ruby 2.0.0 and on, since adding support to something like 1.9.3 is harder and requires rewritting stuff I'll leave that as another issue and leave the PR for this with support of Ruby 2.0.0