Determine the minimum version of Go required for runtime

[paulsmith]

It would be good to let users of Pushup know what is oldest version of Go is that they need to install in order to run pushup build or pushup run. One way to figure this out would be to look at the code generator and some generated .up -> .go files, and see if there are any obvious features that stand out as being introduced with a certain version of Go. The other way would be to systematically build a Pushup project with Go versions starting at 1.0, if it doesn't compile, increment by a point release until it does.