Does not build with target wasm

Following go's [webassembly guide](https://github.com/golang/go/wiki/WebAssembly), the package does not build for wasm. Google's certificate-transparency-go dep is a little too out of date.

```bash
$ GOOS=js GOARCH=wasm go build ./cmd/...
# github.com/google/certificate-transparency-go/x509
/home/j/go/pkg/mod/github.com/google/certificate-transparency-go@v1.0.21/x509/cert_pool.go:38:9: undefined: loadSystemRoots
/home/j/go/pkg/mod/github.com/google/certificate-transparency-go@v1.0.21/x509/root.go:21:32: undefined: loadSystemRoots
/home/j/go/pkg/mod/github.com/google/certificate-transparency-go@v1.0.21/x509/verify.go:852:3: not enough arguments to return
/home/j/go/pkg/mod/github.com/google/certificate-transparency-go@v1.0.21/x509/verify.go:852:11: c.systemVerify undefined (type *Certificate has no field or method systemVerify)
# github.com/lib/pq
/home/j/go/pkg/mod/github.com/lib/pq@v0.0.0-20180201184707-88edab080323/conn.go:321:13: undefined: userCurrent
```