Fix Drone CI.

This commit ignores the TestBucket_Put_ValueTooLarge test on
Drone CI because the containers do not have enough memory. This
test will still be run by anyone else who runs the test suite so
that will still provide coverage.

bucket_test.go

@@ -642,6 +642,10 @@ func TestBucket_Put_KeyTooLarge(t *testing.T) {
 
 // Ensure that an error is returned when inserting a value that's too large.
 func TestBucket_Put_ValueTooLarge(t *testing.T) {
+	if os.Getenv("DRONE") == "true" {
+		t.Skip("not enough RAM for test")
+	}
+
 	db := NewTestDB()
 	defer db.Close()
 	db.Update(func(tx *bolt.Tx) error {