Use environment variables for travis, remove redundant test gemfiles
Fix signature verification for external messages.

Mail::Body.encoded returns an altered string compared to the original if
the original wasn't created with Mail.
Ignore temporary files by vim
Ignore 'tags'-file.
Pin minitest to version 4.

Minitest v5 doesn't work with test/unit:
<https://github.com/seattlerb/minitest/issues/283>.
Merge pull request #15 from duckdalbe/fix-signature-verification

Fix signature verification for external messages.
Merge pull request #13 from meineerde/feature/cleanup_travis

Use environment variables for travis, remove redundant test gemfiles
Merge pull request #16 from duckdalbe/master

Pin minitest to v4, and ignore vim+ctags-files.
pass GMGME signature verification results all the way up
initialize Mail#verify_result after signature check of unencrypted mail