Revert 'go get' command to use gemalto instead of thalesgroup

[rlenferink]

The go.mod file defines the module as github.com/gemalto/helm-spray. Changing Go module paths without making breaking changes is not supported (yet):
golang/go#30831

Therefore this PR to revert to github.com/gemalto/helm-spray, as that still works as well.

Tests:

[root@centos8 git]# go get github.com/gemalto/helm-spray@master
go: downloading github.com/gemalto/helm-spray v0.0.0-20200317081907-8edada558592
go: github.com/gemalto/helm-spray master => v0.0.0-20200317081907-8edada558592
go: downloading k8s.io/helm v2.15.2+incompatible
go: downloading github.com/spf13/pflag v1.0.3
go: downloading k8s.io/apimachinery v0.0.0-20191025225532-af6325b3a843
go: downloading gopkg.in/yaml.v2 v2.2.4
[root@centos8 git]#

[root@centos8 git]# go get github.com/thalesgroup/helm-spray@master
go: downloading github.com/thalesgroup/helm-spray v0.0.0-20200317081907-8edada558592
go: github.com/thalesgroup/helm-spray master => v0.0.0-20200317081907-8edada558592
go get: github.com/thalesgroup/[email protected]: parsing go.mod:
        module declares its path as: github.com/gemalto/helm-spray
                but was required as: github.com/thalesgroup/helm-spray
[root@centos8 git]#

An alternative could be to update the go.mod to use github.com/ThalesGroup/helm-spray. However, that'd be a breaking change and users need to be informed. The easiest solution would be correction the instructions in the README.

/cc @cvila84 @jroucheton