FIX to not use double-quotes when not needed.

Prefer single-quoted strings when you don't need 
string interpolation or special symbols such as \t, \n, ', etc.

Affected [describe, it, story, context, it_should, mock]

Now context makes 
```
context 'title of ths context' do
    # test something...
end
```
instead of 
```
context "title of ths context" do
    # test something...
end
```

Based on: https://github.com/bbatsov/ruby-style-guide#strings