Remove bad advice

[pboling]

Trick to not need to add bundle exec to commands if you add 2 lines to the Rakefile is flawed.

Multiple versions of rake

It does not help at all if you have multiple versions of rake installed.

∴ rake
rake aborted!
Gem::LoadError: You have already activated rake 12.0.0, but your Gemfile requires rake 11.2.2. Prepending `bundle exec` to your command may solve this.
∴ gem list rake

*** LOCAL GEMS ***

rake (12.0.0, 11.2.2, 11.1.2, 10.5.0, 10.4.2, 10.1.0, 10.0.4, default: 0.9.6)

Different Package Manager

It prescribes rubygems which is but one among several options for package managers.

Other use cases

Not all roads lead through the Rakefile, and sometimes bundle exec may still be required. Better to keep things consistent. If it is trouble to type so much shell aliases are an alternative.