adds did-you-mean gem to dev & test groups

This is quite useful during dev & test, most obviously when in the irb console w/in msfconsole etc.

e.g.

```Ruby
>> framework.init_module_path
NoMethodError: undefined method `init_module_path' for #<Msf::Framework:0x007faf8427c8c0>
     Did you mean? #init_module_paths
                              #init_module_paths

>> ::Msf::Util::EXE.is_eicar_corrupted
NoMethodError: undefined method `is_eicar_corrupted' for Msf::Util::EXE:Class
     Did you mean? .is_eicar_corrupted?
                              .is_eicar_corrupted?
```

Don't forget to bundle install.  I guess verification is:

-[ ] bundle install
-[ ] msfconsole -x irb # or tools/msf_irb_shell.rb
-[ ] run framework.init_module_path and see the "did you mean init_module_paths" error message