Compatibility with GnuPG 1.x and 2.x, auto-detect GnuPG version

* aptly can sign and verify without issues with GnuPG 1.x and 2.x
* aptly auto-detects GnuPG version and adapts accordingly
* aptly automatically finds suitable GnuPG version

Majority of the work was to get unit-tests which can work with GnuPG 1.x & 2.x.
Locally I've verified that aptly supports GnuPG 1.4.x & 2.2.x. Travis CI
environment is based on trusty, so it runs gpg2 tests with GnuPG 2.0.x.

Configuration parameter gpgProvider now supports three values for GnuPG:

* gpg (same as before, default): use GnuPG 1.x if available (checks gpg, gpg1),
otherwise uses GnuPG 2.x; for aptly users who already have GnuPG 1.x
environment (as it was the only supported version) nothing should change; new
users might start with GnuPG 2.x if that's their installed version

* gpg1 looks for GnuPG 1.x only, fails otherwise

* gpg2 looks for GnuPG 2.x only, fails otherwise

pgp/keyrings/aptly2.pub.armor

@@ -0,0 +1,30 @@
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+
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+=Tu5z
+-----END PGP PUBLIC KEY BLOCK-----