Remove whitespace from Cheffile in integration tests
Support :librarian option in `solo bootstrap`

PR #180 added `--no-librarian` option to the cook command. If the
bootstrap command does not know about the key, the value will be set to nil
thus disabling `librarian-chef install`. This fixes it (and integration
tests.)
Remove and gitignore Gemfile.lock

Let the developers and CI servers always install the newest
dependencies. Also makes it easier to test with different Chef versions.
Test also with Chef 11 on Travis

Make it possible to pass Chef version requirement via environment
variable and use that in Travis.
Fix tests on Chef 11
Default to Chef 11

Change default Chef requirement to allow Chef 11.

Test agains the latest Chef release and latest 10.x version on Travis.