Relax rainbow spec to allow rainbow 3.0.0

[knightq]

The main news are:

• removed Rake dependency
• dropped MRI 1.9.3 and 2.0 compatibility
• added String refinement
• added new Rainbow.uncolor method

Support for MRI 1.9.3 has been already dropped even on pronto.

[doomspork]

@knightq could you provide some additional information? Is there a reason for this change? Should we be looking at updating to 3.0?

[dylanpinn]

@doomspork I am currently getting this error message whilst adding this and the latest reek to my project

    pronto-rubocop (~> 0.9.0) was resolved to 0.9.0, which depends on
      pronto (~> 0.9.0) was resolved to 0.9.5, which depends on
        rainbow (~> 2.1)

    reek (~> 4.8) was resolved to 4.8.0, which depends on
      rainbow (~> 3.0)