build(): make FF beta optional until SL fixes it.

Author: jelbourn

test/browser-providers.ts

@@ -28,7 +28,7 @@ const configuration: { [name: string]: ConfigurationInfo } = {
   'Chrome':       { unitTest: {target: 'SL', required: true}, e2e: {target: null, required: true}},
   'Firefox':      { unitTest: {target: 'SL', required: true}, e2e: {target: null, required: true}},
   'ChromeBeta':   { unitTest: {target: 'SL', required: false}, e2e: {target: null, required: true}},
-  'FirefoxBeta':  { unitTest: {target: 'SL', required: true}, e2e: {target: null, required: true}},
+  'FirefoxBeta':  { unitTest: {target: 'SL', required: false}, e2e: {target: null, required: true}},
   'ChromeDev':    { unitTest: {target: null, required: true}, e2e: {target: null, required: true}},
   'FirefoxDev':   { unitTest: {target: null, required: true}, e2e: {target: null, required: true}},
   'IE9':          { unitTest: {target: null, required: false}, e2e: {target: null, required: true}},