use travis-ci build matrix to test different compilers

- fixes #734

- dmd-2.065.0, dmd-2.066.1, ldc-0.14.0, gdc-4.9.0
workaround Issue 13194 in 2.065 compiler

- ICE when initializing static class members to void

- the static variable would probably end up in the binary
  even though it was only used to check for a property

https://issues.dlang.org/show_bug.cgi?id=13194
workaround ICE in ldc-0.14.0

- ldc segfaults while building a nested context for ClientTask.execute
  function (not sure why it does build a context)

- apparently fixed in ldc-0.15.0-beta1
workaround GDC issue with undefined lambda

- the lambda is emitted to libvibe-d.a but only
  as local symbol (small t in nm)

- fixed by templatizing the auto function
  (lambda is generated on first call)
drop sudo for testing (allows to run on docker)
use Travis-CI's new D support
Merge pull request #901 from MartinNowak/travis-ci

Use travis-ci build matrix to test different compilers