Fix tests: mark xfailing ones, add Makefile and Travis

[blueyed]

You probably have to enable Travis integration.

[blueyed]

Builds can be seen here: https://travis-ci.org/blueyed/detectindent/builds

[blueyed]

'check that all four options are changed to a correct value' is failing on Travis' old Vim: https://travis-ci.org/blueyed/detectindent/builds/173735302#L187.

[blueyed]

Stopping there for now.
For Neomake we're using a matrix of different Vim versions, so this could be useful for this repo later, too.
(https://github.com/neomake/neomake/blob/master/.travis.yml)