Drop support for go1.7
Drop support for go1.7

Newer apimachinery fails to build on go 1.7:

```
vendor/k8s.io/apimachinery/pkg/apis/meta/fuzzer/fuzzer.go:144: c.Uint64 undefined (type fuzz.Continue has no field or method Uint64)
vendor/k8s.io/apimachinery/pkg/apis/meta/fuzzer/fuzzer.go:156: c.Uint64 undefined (type fuzz.Continue has no field or method Uint64)
```

anguslees/sealed-secrets