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.
Support different Chef versions in integration tests

If CHEF_VERSION is set, then the local Knife's version is forced to the
omnibus installer.
Test the latest Chef also with Ruby 2.0.0 (rc)
Loosen net-ssh restriction

New Chef releases depend on net-ssh `~> 2.6`.
Fixes #188.
Make sure Chef version parsing works even with sudo prompts

The same fix than in commit 01ce24a8 for #190.

Maybe this should be DRY-ed up, but probably not a big deal for now.
Merge pull request #183 from tmatilai/chef-11

Support Chef 11