Enable sass by default, as docs state

* If `options.sassLint` is omitted it is null and falsy.
* The default state was therefore no sass linting
* Explicitly check that it has been set to false so that if the option
is not provided sassLint runs

modules/govuk_jenkins/files/var/lib/jenkins/groovy_scripts/govuk_jenkinslib.groovy

@@ -146,7 +146,7 @@ def buildProject(Map options = [:]) {
       echo "WARNING: You do not have Ruby linting turned on. Please install govuk-lint and enable."
     }
 
-    if (hasAssets() && hasLint() && options.sassLint) {
+    if (hasAssets() && hasLint() && options.sassLint != false) {
       stage("Lint SASS") {
         sassLinter()
       }