Go 1.10: memcache_test.go:45: Skip call has possible formatting directive %s

[nim-nim]

1031fa0 does not pass unit test with Go 1.10

+ GOPATH=/builddir/build/BUILD/memcache-1031fa0ce2f20c1c0e1e1b51951d8ea02c84fa05/_build:/usr/share/gocode
+ go test -buildmode pie -compiler gc -ldflags '-extldflags '\''-Wl,-z,relro  '\'''
# github.com/rainycape/memcache
./memcache_test.go:45: Skip call has possible formatting directive %s
FAIL    github.com/rainycape/memcache [build failed]