Fix for when Go is used from source without release tag

[ramya-rao-a]

Fixes #545

• Moved getGoVersion, SemVersion and isVendorSupported from goInstallTools.ts to util.ts
• check for GO15VENDOREXPERIMENT env var is added for Go 1.6 because

Go 1.5 introduced experimental support for vendoring, enabled by setting the GO15VENDOREXPERIMENT environment variable to 1. Go 1.6 keeps the vendoring support, no longer considered experimental, and enables it by default. It can be disabled explicitly by setting the GO15VENDOREXPERIMENT environment variable to 0. Go 1.7 will remove support for the environment variable.

• If no release tag is found and GO15VENDOREXPERIMENT is explicitly set to 0, isVendorSupported will return false.
• If no release tag is found and GO15VENDOREXPERIMENT is not explicitly set to 0, isVendorSupported will return true.
• If release tag is not found, golint and gotests would be installed assuming that the Go version supports them. If not, then error messages from running these tools will bubble up to the user