Update coveralls to version 0.8.15

Author: sferik

Diff for: Gemfile.lock

@@ -49,12 +49,12 @@ GEM
       execjs
     coffee-script-source (1.10.0)
     concurrent-ruby (1.0.2)
-    coveralls (0.8.14)
+    coveralls (0.8.15)
       json (>= 1.8, < 3)
       simplecov (~> 0.12.0)
       term-ansicolor (~> 1.3)
       thor (~> 0.19.1)
-      tins (~> 1.6.0)
+      tins (>= 1.6.0, < 2)
     crack (0.4.3)
       safe_yaml (~> 1.0.0)
     devise (4.2.0)