[mrbgems] GEM Dependency Check

__Follow up to:__
#1229 First strik on GEM Dependencies
#1233 [mrbgems] Identify GEM duplications

This pull request is implementing the GEM dependency version check. The following options are implemented:

* = (version is identical)
* != (version is not equal)
* < (version has to be less)
* > (version has to be greater)
* <= (version has to be less or equal)
* >= (version has to be greater or equal)
* ~> (pessimistic version check - twiddle-wakka)

Usage is as specified in #1229

```
MRuby::Gem::Specification.new('mruby-gtk') do |spec|
  spec.license = 'MIT'
  spec.authors = 'mruby developers'
  spec.version = '1.4.1'

  # user information that this GEM depends on GTK
  spec.requirements << "GTK+ >= 3.8.0"
end

MRuby::Gem::Specification.new('mruby-gui') do |spec|
  spec.license = 'MIT'
  spec.authors = 'mruby developers'

  # add GEM dependency mruby-gtk
  # Version has to be between 1.0.0 and 1.5.2
  spec.add_dependency('mruby-gtk', '>= 1.0.0', '<= 1.5.2')
end
```