Mark go.mod as requiring at least 1.13

```shell
[jsonp@redacted]:~/go-attestation> go get -u github.com/google/go-tpm/tpm2@latest
go: finding github.com/google/go-tpm/tpm2 latest
go: finding github.com/spf13/cobra v1.0.0
go: finding github.com/stretchr/testify v1.5.1
go: finding github.com/golang/protobuf v1.4.1
go: finding github.com/spf13/viper v1.6.3
go: finding github.com/google/go-tpm-tools v0.1.2
go: finding github.com/fsnotify/fsnotify v1.4.9
go: finding github.com/cpuguy83/go-md2man/v2 v2.0.0
go: finding github.com/pelletier/go-toml v1.7.0
go: finding github.com/armon/consul-api latest
go: finding gopkg.in/yaml.v2 v2.2.8
go: finding github.com/coreos/go-systemd latest
go: finding golang.org/x/crypto latest
go: finding gopkg.in/check.v1 latest
go: finding github.com/tmc/grpc-websocket-proxy latest
go: finding golang.org/x/sys latest
go: finding github.com/coreos/etcd v3.3.20+incompatible
go: finding github.com/russross/blackfriday/v2 v2.0.1
go: finding github.com/coreos/bbolt v1.3.4
go: github.com/coreos/go-systemd@v0.0.0-20200316104309-cb8b64719ae3: go.mod has post-v0 module path "github.com/coreos/go-systemd/v22" at revision cb8b64719ae3
go: finding golang.org/x/net latest
go: finding github.com/shurcooL/sanitized_anchor_name v1.0.0
go: finding github.com/json-iterator/go v1.1.9
go: finding github.com/coreos/etcd v3.3.13+incompatible
go: finding google.golang.org/protobuf v1.22.0
go: github.com/coreos/bbolt@v1.3.4: parsing go.mod: unexpected module path "go.etcd.io/bbolt"
go get: error loading module requirements
```

I'm a little confuzzled, but I'll see what I can work out. If anyone has any ideas let me know.