Fix source 6 warnings

[ivanpauno]

Replaces #66 (I opened that one in a fork by accident).

I hope this workflow config works 😬

[ivanpauno]

If we need to specify -target -source in Android as commented here, we can readd that in the future.

I'm using --release in the github workflows, because it does the right thing TM.
See https://stackoverflow.com/questions/43102787/what-is-the-release-flag-in-the-java-9-compiler.

[ivanpauno]

mmm, not sure why the build failed, I'm going to double check

[ivanpauno]

Well, actually our code is not fully compatible with java 6 (java.time.Duration doesn't exist).
--release 6 is more strict that passing -source 1.6 -target 1.6.

I will use the later, because I think that java.time.Duration is available in android anyways ... (either through the ThreeTen backport or through the android gradle 4.0 plugin).

[ivanpauno]

CI now looks ok (same failure that we were having before and that #70 should fix 😃 )

[jacobperron]

I've rebased the changes.

[jacobperron]

#76 should fix the infrastructure issue.

[ivanpauno]

Rebasing again after #76 was merged.