Changelog for MiniAgda-0.2019.12.13

CHANGELOG for MiniAgda 0.2019.12.13 Tested compilation with ghc 8.8.1. 0.2019.3.29 Bumped haskell-src-exts to 1.21. 0.2018.11.6 Tested compilation with ghc 8.6.2. 0.2018.11.4 Fixed compilation with ghc 8.4 and 8.6. 0.2017.02.18 Updated install instructions. 0.2016.12.19 Travis instance ensures compilation with ghc 7.6 -- 8.0. 0.2014.9.12 Internal changes: * Removed dependency on IfElse. * Use ExceptT instead of ErrorT, depend on mtl > 2.2. This also required to switch from fail to throwError for catchable errors.