check builds with multiple compilers

[Samael]

please consider updating your .travis.yml to ensure that the project will work with various os/compiler combinations.

language: d

sudo: required
dist: trusty

d:
  - dmd
  - dmd-2.069.2
  - ldc
  - gdc

os:
  - linux
  - osx

[Gary Willoughby]

I've updated the travis file with the following:

language: d
sudo: false

d:
  - dmd
  - gdc
  - ldc

GDC seems to have some issues on OSX on travis so i've omitted the os part.