Support for go 1.9

[coryb]

Hello,

I am using xgo to build and release several of my projects and would like to start using go 1.9 features and fixes, unfortunately when I try with xgo --go 1.9 I get:

Checking for required docker image karalabe/xgo-1.9... not found!

I am not sure what is involved in getting a new xgo supported version, hopefully it is not too hard, or perhaps you can point me in a direction to help provide you a PR?

Thanks for xgo, it has been extremely useful!

[adg]

Is there anything I can do to help push this forward?

[lixingwang]

+1, waiting for the PR to be merged, please make it forward.