Updated build for hledger-lib version 0.18.
Updated build for hledger-lib version 0.19.3.
.travis.yml: enable automatic regression testing by travis-ci.org

Not that we have any tests ... but it will verify the build, too.
release.nix: add build instructions for continuous integration with Hydra
.gitignore: ignore internal cabal-dev files
Hledger/Interest.hs: adapt to latest version of hledger-lib

What used to be 'teffectivedate' is now called 'tdate2'.
Furthermore, Postings now have an (optional) date stamp, too.
release.nix: run builds with ghc 7.4.2 and 7.6.2
release.nix: drop unused 'officialRelease' argument
Merge changes from branch 'v1.4.x'.