Add version check

[ghost]

Fixes: #68

"unstable" seems to be needed if phpBB is in debug mode: cyberalien/sass_compiler@e332ffd

[ghost]

CI is failing because of:

  [RuntimeException]                                                           
  The openssl extension is required for SSL/TLS protection but is not availab  
  le. If you can not enable the openssl extension, you can disable this error  
  , at your own risk, by setting the 'disable-tls' option to true.             

Probably because of the https links in composer?

[ghost]

@rxu Any opinions about that? I think this is a minor change with shouldn't show any issues in the current state of the extension.

[rxu]

Closed with 82693c0.