Add a means to test dpl from github
Add a means to test dpl from github

With the following in .travis.yml:

deploy:
  provider: X
  edge:
    branch: foo
  ⋮

we will build and install dpl gem from
the branch 'foo' of https://github.com/travis-ci/dpl.

This allows more flexible testing of dpl PRs without merging
them to the master branch.

andyli/travis-build,luoqii/travis-build,Acidburn0zzz/travis-build,tmccombs/travis-build,kidaa/travis-build,craigcitro/travis-build,akoeplinger/travis-build,gavioto/travis-build,tmccombs/travis-build,lrowe/travis-build,Acidburn0zzz/travis-build,dirk/travis-build,JuliaCI/travis-build,akoeplinger/travis-build,jhass/travis-build,JuliaCI/travis-build,Tiger66639/travis-build,kevmoo/travis-build,andyli/travis-build,Tiger66639/travis-build,gavioto/travis-build,dirk/travis-build,craigcitro/travis-build,jhass/travis-build,luoqii/travis-build,lrowe/travis-build,tianon/travis-build,kevmoo/travis-build,kidaa/travis-build,tianon/travis-build