fix broken test case

- linenumbers-all.css had a hard-coded path instead of a parameterized one
Merge pull request #1844 from mouyang/travis

fix broken test case
Added support for property merge via `+_`;
Property merge via `+_`: minor test updates.
Fix double toCSS calls in unusual situations causing an exception. Fixes #1830
Merge branch 'master' of https://github.com/cloudhead/less.js
1.6.3 release
Merge pull request #1847 from seven-phases-max/property-merge-with-space

Property merge with `+_` (replaces #1788)