Ensure UTF-8 is used for reading/writing files

Play 2.2 and earlier added -Dfile.encoding=UTF-8 to the build script.
Activator does not, which means we can't use this to enforce UTF-8
everywhere, so now we need to be explicit about UTF-8.
Merge pull request #2862 from pvlugter/2838-explicit-utf-8-2.3.x

Ensure UTF-8 is used for reading/writing files (backport 2.3.x)