Supportability of GO111MODULE=OFF for external integrations

[rhcarvalho]

When GO111MODULE=OFF, prevent Travis from trying to build external integrations fetched with "go get".

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While we want to make sure the core of sentry-go can be used with or without Go Module mode enabled, (at least some) integrations with external frameworks have different requirements. As an example, the Iris framework is meant to be used with Module mode on.

In particular, when GO111MODULE=OFF, the Travis build does a "go get" and fetches all of the latest versions of all transitive dependencies of sentry-go and integrations.

That way of fetching packages must not be in Travis, as it is impractical to maintain. See #135.

In practice, users of sentry-go may use sentry-go and its integrations with compatible versions of the frameworks involved, and they may use whatever method they like to track versions: vendoring, modules, dep, glide, ...