README: update minimum go version (#2923)
README: update minimum go version (#2923)

PR #2888 bumped the minimum go version from 1.12 to 1.13 but did not
update README.md to reflect the change. This commit updates the readme
to be in line with the code.

Attempting to build `mc` from source on go 1.12 yields this error:
```
$ GO111MODULE=on go get github.com/minio/mc
go: finding github.com/minio/mc latest
package github.com/minio/mc: build constraints exclude all Go files in /home/andy/go/pkg/mod/github.com/minio/mc@v0.0.0-20191004065813-f79aefa79713
```
minio/mc,balamurugana/mc,vadmeste/mc,minio/mc,vadmeste/mc,krisis/mc,krisis/mc,balamurugana/mc