feature: test interoperability of gpg on automated builds

[joshrabinowitz]

The idea is to test the interoperation of git-secret (and version of gpg used on that build) between some or all of the builds we use for automated builds/tests on travis-ci.

Could involve a second repository shared between builds which records which version of gpg, on which system type, did each operation on each file. Might have a separate sub-directory for each build type or buildtype-to-buildtype combination.

[sobolevn]

That would be awesome.

Related to #200

[joshrabinowitz]

Another way to approach this is, on distributions where both a version of gnupg1 and gnupg2 are packaged, perform tests across the two available versions of gpg on that system. This will allow for our self-contained tests to verify gnupg interoperability using just a single OS with no external pieces.