Remove Guava dependency

[LeoNiedermeier]

Is your feature request related to a problem? Please describe.

The library includes guava, but uses just two classes (so far I can see)

• Precondition
• ImmutableMap

Both can be replaced with plain java classes, e.g

• Precondition => java.util.Objects#requireXYZ methods

Describe the solution you'd like

No guava dependency because

• you use just few classes from guava which can replcaed by core java classes
• version conflicts, because other dependencies in a application which uses snowplow can also include guava with different version
• as user of this library i do not want additional transitive dependencies in my application

Describe alternatives you've considered

Additional context

[mscwilson]

Hi @LeoNiedermeier, thanks for the suggestion. We'll look into this for the next release.

[mscwilson]

Hi @LeoNiedermeier. We just released Java tracker v1. Guava has been removed 😊