Integrate composer in the Travis build process

Creates a package definition from the framework version being built,
and uses composer to install it into an installer project, as well
as the required modules for testing.
Update versions used in the Travis build.
Merge pull request #1238 from ajshort/travis-composer-3.0

Travis Composer Integration (3.0.x)

Conflicts:
	tests/travis/before_script
API Remove content-length setting in HTTPResponse

It's not reliable. Started in c69381c33, but only partially reverted.
Using composer.lock for LeftAndMain->CMSVersion()

See https://github.com/silverstripe/silverstripe-cms/pull/289 for context