[FEATURE] Adopt `quoted-strings` rule from `yamllint` for selected files [ssf]

[myii]

Is your feature request related to a problem?

No.

Describe the solution you'd like

Use the quoted-strings rule, only for .travis.yml and .yamllint for the time being.

I've adopted this throughout the ssf-formula, since I ran into some YAML ambiguity errors. It would make things easier if these two files are managed here as well.

Actually, the changes are pretty much complete already in the files themselves, it's just a matter of adding the rules to the .yamllint file.

Describe alternatives you've considered

N/A.

Additional context

[aboe76]

@myii do you have an example of the output?

[myii]

There won't be any difference to the output, all this is doing is ensuring that the YAML we're using in these two files is more robust (strings are always quoted).

[aboe76]

Ok I thought there would be output from the yamllint itself...