Resolve "http.StatusTooManyRequests breaks Go 1.5 compatibility"

This merge request replaces a constant introduced in go1.6 with its literal value, as we need go1.5 support. No other changes are required.

It also modifies .gitlab-ci.yml to test against go1.5, 1.6 and 1.7, to prevent future regressions.

Closes #69 (closed)