Update Facebook color.

Updated Facebook SSO button color acccording to their brand
guidelines.

VAN-296

lms/static/sass/partials/lms/theme/_variables-v1.scss

@@ -186,7 +186,7 @@ $ui-notification-height: ($baseline*10);
 
 // social platforms
 $twitter-blue: #55acee;
-$facebook-blue: #4267b2;
+$facebook-blue: #1877F2;
 $facebook-focus-blue: #29487d;
 $linkedin-blue: #0077b5;
 $google-blue: #4285f4;