Add /v2 suffix to import (oklog#47)

If we opt-in to modules with a non-v0 or v1 version, we have to
explicitly append /vX to the import path whenever we import.

This is infuriating.

go.mod: put v2 in the module path

TestMonotonicSafe fails sometimes (oklog#44)

* TestMonotonicSafe fails sometimes

* TestMonotonicSafe: vary seed and inc

* Check for MonotonicReader interface instead of *monotonic

* rm seed/inc variation in test, it's not relevant

* TestMonotonicSafe should use multiple goroutines

* t.Fatalf must be called in the same goroutine as the test

Release 1.3.1

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Release 1.2.0

Release 1.1.0

Release 1.0.0

Release v0.3.0

Release v0.2.0