fix breaking test and remove unused lib.

Author: Mike Ng

packages/optimizely-sdk/lib/optimizely/index.tests.js

@@ -2911,7 +2911,7 @@ describe('lib/optimizely', function() {
 
     describe('#getEnabledFeatures', function() {
       beforeEach(function() {
-        sandbox.stub(optlyInstance, 'isFeatureEnabled', function(featureKey) {
+        sandbox.stub(optlyInstance, 'isFeatureEnabled').callsFake(function(featureKey) {
           return featureKey === 'test_feature' || featureKey === 'test_feature_for_experiment';
         });
       });

packages/optimizely-sdk/package-lock.json

@@ -49,7 +49,6 @@
     "karma-mocha": "^1.3.0",
     "karma-sinon": "^1.0.5",
     "karma-webpack": "^2.0.3",
-    "mathjs": "^3.3.0",
     "mocha": "^5.2.0",
     "mocha-lcov-reporter": "^1.3.0",
     "nock": "^10.0.1",