install via curl fails on arm64 platform.

[vielmetti]

I get the following error when I try to install gometalinter on an arm64 system.

% curl -L https://git.io/vp6lP | sh
alecthomas/gometalinter crit platform linux/arm64 is not supported.  Make sure this script is up-to-date and file request at https://github.com/alecthomas/gometalinter/issues/new

I am not sure how to reconcile this with the change at #553 which adds arm64 binaries.

[vielmetti]

The impetus for this is a port of loki, see grafana/loki#67 .

[alecthomas]

Fixed!