Travis: add ppa to install cmake 2.8.11 required by tidy-html5
Travis: add ppa to install cmake 2.8.11 required by tidy-html5

sk-/git-lint,sk-/git-lint,stfp/git-lint,sk-/git-lint,sk-/git-lint,sk-/git-lint,sk-/git-lint,stfp/git-lint,stfp/git-lint,sk-/git-lint,stfp/git-lint,stfp/git-lint,stfp/git-lint,stfp/git-lint,sk-/git-lint,sk-/git-lint