Rescue Redis' CannotConnectError, the algorithm should allow it.

Fixes #60.

lib/redlock/client.rb

@@ -112,6 +112,8 @@ def lock(resource, val, ttl, allow_new_lock)
         recover_from_script_flush do
           @redis.evalsha @lock_script_sha, keys: [resource], argv: [val, ttl, allow_new_lock]
         end
+      rescue Redis::CannotConnectError
+        false
       end
 
       def unlock(resource, val)

spec/client_spec.rb

@@ -140,6 +140,18 @@
       end
     end
 
+    context 'when a server goes away' do
+      it 'does not raise an error on connection issues' do
+        # We re-route the lock manager to a (hopefully) non-existent Redis URL.
+        redis_instance = lock_manager.instance_variable_get(:@servers).first
+        redis_instance.instance_variable_set(:@redis, Redis.new(url: 'redis://localhost:46864'))
+
+        expect {
+          expect(lock_manager.lock(resource_key, ttl)).to be_falsey
+        }.to_not raise_error
+      end
+    end
+
     context 'when script cache has been flushed' do
       before(:each) do
         @manipulated_instance = lock_manager.instance_variable_get(:@servers).first